The only thing I would change is that this makes it seem like powered flight was waiting on the engine, when it was waiting on the plane.
One thing that struck me at the Museum of Flight in Seattle is how far advanced IC technology was while the Wright brothers were gliding in wooden contraptions. They had turbocharged v12 diesel engines and Benz was already making aluminum engines for cars before the Wright brothers made their first attempt.
I found it fascinating how as soon as flight was demonstrated, it only took a few years to go from a wooden prototype to a sophisticated machine with a very complex engine.
Bonus pic of some of those early engines https://i.imgur.com/mENnuHH.png
And of course even if you survive the iterative development process, all you’ve got is a (largely) useless glider until the engine comes along and you can commercialize it.
It seems to me that for flight the construction materials (silk, bamboo, string, glue - what the early WW1-era aircraft were made from) and the propulsion (solid-fuel rocket) have been around for over a thousand years.
The original Chinese designs relied on gunpowder. This is self-oxydising and burns rapidly, but has a comparatively low power density, roughly 1/10th that of liquid petroleum fuels.
Liquid fuel such as alcohol and oil existed, but the notion let alone the availability of oxidizers didn't until the early 19th century. The first use of liquid oxygen in rockets didn't occur until 1926, by Robert Goddard.
Hypergolics or solid rocket motors would have been other options, but both are still pretty advanced. I've no idea how likely they'd have been.
One of the more viable solid fuels might have been rocket candy, made of sugar and usually potassium nitrate as an oxidizer. Both would have been available.
Demonstration here: https://piped.kavin.rocks/watch?v=12fR9neVnS8
Whether or not that would generate sufficiently strong and reliable thrust for a steampunk JATO launch, I'm not sure.
Rocket does have the trust ratio needed. But then there is questions of burn time and control. So you could get up there, with some risk. But not stay there for very long time.
Short googling for solid fuel rockets seem that longest burn times are less than 3 minutes... And that is best case scenario with 20th/21st century technology...
It could still have been possible to work things out based on JATO launch, though the hang-glider approach of low sand-dune testing / training would probably have been safer and a better overall option.
Powered aviation also helped drive and prove materials design enhancements and general aviation theory, controls, avionics, etc., all of which transfer well to gliders, but would very likely have been far more difficult to develop in a a glider-only regime.
I'm saying that having an airworthy craft once you're off the ground takes some doing, and it's probably easier to get there if you're experimenting with powered craft.
The early powered heavier-than-air craft were not especially aeronautically sound. But with the ability to perform design-build-test cycles, and not kill overly many pilots in the process, once engines existed design progressed rapidly.
Getting from the Wright Flyer to a modern sailplane would have been far more challenging without engines.
Or land it.
So you build your best guess at what a working glider might be. You don't have robotics or radio to remotely control it, so someone's got to fly the thing.
You build a kite flinger and/or ramp or rail to toss it off the side of a building or cliff or whatever.
And then you hope, desperately, that you weren't too terribly mistaken about a great many things.
Franz Reichelt wasn't quite building airplanes, but he turned out to have misjudged his own competence in a somewhat similar manner:
https://allthatsinteresting.com/franz-reichelt
Now consider the same situation, except that you've at least got a motor to buy your way out of design and/or piloting errors, at least a little bit.
Under which of those scenarios do you think actual useful design might have progressed faster?
Keep in mind that people have had the notion of flying with wing-like contraptions dating to the ancient Greeks (see the legend of Icarus). There were numerous inventors who threw themselves off hills or cliffs with various attempts. Lack of power, and the frequent short professional career track of such inventors tended to stymie progress.
Successful gliders and sailplanes almost entirely postdated powered heavier-than-air flight. The first designs appeared after WWI, and practical use didn't appear until the 1930s. My understanding is that popularity of recreational gliding didn't emerge until the 1950s or 1960s, again benefiting from aeronautical engineering, materials, radios, much better knowledge of aircraft operation, controls, and instrumentation.
Glide ratio is one measure of aerodynamic efficiency and sophistication. Early 1930s gliders achieved about a 1:17 ratio. Most modern gliders exceed 1:30, and the best 1:50 or more. This expresses altitude loss per unit foreward travel (e.g., 1 meter loss for 30 meters forward flight).
Google's Ngram viewer is a somewhat fickle guide, but suggests an initial spike in mentions in the late 1930s / 1940s, again in the 1950s, then a third in the 1970s:
https://books.google.com/ngrams/graph?content=sailplane&year...
Adding in "glider" (multiplied 10x) still lags "aeroplane" by decades.
https://books.google.com/ngrams/graph?content=%28sailplane%2...
There may have been earlier terminology used, and "glider" has other meanings which might confound matches, but at least for "sailplane", the trend line lags "aeroplane" and "airplane" considerably. (I've multiplied "sailplane" results 100x in this plot):
https://books.google.com/ngrams/graph?content=%28sailplane%2...
Once rapid design-build-test cycles were practial, the initial Wright design rapidly shifted to a monocoque fuselage with forward wings and empennage, emerging within a decade.
In just over 30 years after the Wright's first powered flight, the DC-3, an aircraft still in commercial operation was flying. It has been described as the perfection of aircraft design, and the major elements of its design are still present on contemporary aircraft, though of course jet engines have largely replace reciprocating piston engines.
Major factors in successful powered or unpowered flight have been understanding aerodynamics (largely through experimentation and test flights, increasingly through modelling), materials (pre-industrial materials are poorly suited to human-scale aircraft or gliders), controls (both theory and interfaces), powerplants (on powered aircraft).
Ultralights, hang gliders, and sailplanes all benefit greatly from specific materials: Nylon for wings, aluminium for structural members, plastics, and steel for wires and cables. Instrumentation, navigation, communications, transponders (safety) and radar (collision detection) also factor in.
Virtually all of these are dependent on earlier stages of industrialisation: smelting of iron and aluminium, petroleum chemistry and textiles fabrication for Nylon and plastics, earlier aviation engineering for general flight handling and control theory, electronics for instrumentation, radio, and radar, plus domain knowledge from other fields such as physics (instrumentation, controls, etc.).
TL;DR: Prerequisites and path dependencies.