Today Grace Hopper's "compiler" wouldn't merit the name, it's just a linker loader, what's the big deal? Well the big deal was nobody had actually programmed the computer to help them program the computer before. A huge sub-discipline of computer science simply didn't exist.
Consider another discipline, metrology. Pioneers in metrology weren't doing advanced physics like you would see in a metrology lab today, they just had the insight that it sure is easier if everybody agrees how much corn this is, so they used prototypes, which is pretty much the simplest strategy but it's already a huge improvement on nothing at all. It took until this century for the last prototype (the Kilogram) to be replaced. On the route there those prototypes got a lot more sophisticated, and we learned a lot more about what it actually means to have a certain amount of a thing, so metrology got much harder.
The other surprising thing to me was that it didn't really resemble modern algebra at all. As a former math student I think it's easy to create a mental model of math history where when new fields are created they resemble the modern incarnation. But when you actually look at the primary founding texts its clear the ideas and notation are pretty unrefined at least compared to today's standards.
I've just started reading it and Petzold claims all you need is a decent grasp of high school mathematics.
Glad to see I'm not the only one...