I run out of gears on flat terrain but that is a negligible portion of what I ride. I'd definitely expect a gravel bike to be a bigger handicap if you're riding mostly flat terrain and actually care about your speed on those sections.

Serious gravel racers definitely run more "road-like" ratios

If you can spin out a 50x11 or similar on the flat you're in a very small minority of cyclists...

I'm not spinning out but I am spinning faster than I can sustain / is efficient. I'm looking for the next gear basically. Spinning out is much faster and requires and descent (for me at least). Based on Strava I'm not that strong. Top 40% maybe. If I was racing I'd be cat 4. My observation is more to do with stronger riders than I am. Once you get to a certain speed/ftp all the considerations like rolling resistance, gearing, aero etc become much more obvious. If you are averaging 29.5km/h like in the test you might think optimisations don't make much difference but if you average 40kmh+ you will know they do.

They even note this in the article "If you ride much faster, then it’s possible that wider tires have a little more wind resistance, but the difference will be so small that it’ll get lost in all the other factors that influence your bike’s speed. On the other hand, if you ride slower, then the advantage of wider tires will be even greater."

I don't think the difference is that large, but there is some. I haven't tried myself, but would be interesting to test it against each other.

You can compare gearing ratios on 99spokes, e.g. https://99spokes.com/compare?bikes=scott-speedster-gravel-20...

I've been road riding with people who switched to gravel bikes and they were suddenly noticeably slower. They also seemed to run out of gas earlier which means they were expending more energy to keep up.

Weren't those new bikes heavier?

My own road bike is nothing particularly fancy, but it's very light. Just because those bikes may be made to take more of a beating than a road-bike, they may be heavier. I'm thinking in particular tougher wheels and front work, possibly the whole frame.

We were riding on mostly flat terrain so it wouldn't be a big differentiator.

Were they using tires without any tread?

interesting, maybe different frame geometry contributed as well?

I would imagine this would be considerable. Improving your aero position is the easiest way to go faster.

My 2019 Trek Crossrip has 48/32 and 11-32. Of course, you could switch those components up - the biggest difference between gravel and road is the clearance for wider tires, and "relaxed" or upright geometry, plus capability for more mounting.

And Cannondale even has a carbon frame with a front suspension and rear flex on the frame for a near-full-suspension behavior.

Not much smaller, 46/30 crank instead of 50/34 and maybe 34 or 36 rear cogs instead of 28 or 32.

You could put a road groupset on most gravel frames.