I do agree with most of your post, and the whole your average cyclist doing “30 mph without much trouble” is ridiculous. I do think you are underestimating how fast road bikes can be though.
You are not reaching 30mph unless you are fully sprinting on a descent with a gravel bike (maybe a mountain bike if it’s a long, long, stretch) or have a road bicycle on a flat/slight slope and you are full sending it (even on a flat road I’m assuming, I’ve never ridden one). Not to mention these people will be using protective gear
So, on a road bike, it is pretty easy on the flat to keep 20mph/32kmh. 30mph/48kmh is definitely an effort and not one that is sustainable for most people. To give you an idea, I did a charity bike ride in June which was just shy of 100 miles, it was 158.3km with 1667m of climbing as well, so not completely flat. I averaged 31.2kmh. I am in no way fast, I am alright on the flat but gravity is a cruel mistress on an uphill.
Downhill is a different thing as well, I have hit speeds of around 62mph/100kmh, and a hill near me will almost always spit me out doing 55mph/89kmh with 0 effort(roll down the hill and sit on the drops), and that is on both my road and gravel bikes. But that is probably steeper than you are thinking. However, there is also a pretty steady -1% average “downhill”, it is a false flat, but it is really easy to cruise along at 25mph/40kph+.
I think my biggest issue with this thread is the bikes they are referring too are actually Electric Motorbikes(and should be treated like any other motorbike), not an “ebike” in the typical sense of Pedal Assist Bicycle.