Fair enough and point taken. However, this isn't riding a bike on a trainer indoors. Or even running in a line on a track.
Multi-day/week racing, outside, variable road/weather conditions, 100+ people, numerous teams with differing goals, break aways, new routes and new parcours.
Agreed, a lot more going on with any and all "ball sports," but there is plenty more going on than just a fitness test when racing GTs at the pro level. If it was just about fitness, your solid triathletes would be out there snatching up wins at kermesses in Belgium.
And just look at some of the guys who are top-level, but don't win that often. I'd say there is more going on than just fitness (PEDs or not) and pro cycling.
In my eyes road cycling is a sport that doesn't necessarily require that much skill in order to become a pro or even world tour pro. That's the difference between cycling and most other sports which you need to take up before you are ten, eight, or six. If you have a great physiology, some good balance and reaction times, you can compete. However, winning races, and winning races regularly, is something else.
To do that real skill, meaning automatisms in your reactions, behaviour, coordination, is incredibly helpful, up to necessary, and such automatisms can only be gained in your younger years.
I would still differentiate between several skills on the bike, though - I think to be able to balance your bike in almost every situation is something you cannot learn in your mid-twenties. It's partly a question of inherited coordination and partly, and mostly, training at young age.
On the other hand improving your descending skills by a lot seems pretty possible, because it's in addition to natural skill very much, I think, a question of getting used to it, a mental thing, and a cognitive thing - learn how to find the best line.
Riding and behaving best in a peloton, positioning, is also something you can still learn later on, but it is way easier if you have done it for many years in your youth.
In comparison to many other sports cycling doesn't require the same amount of technical skill. You can start it in your late teens or early twenties if you have some good basic coordination and such. But we often make the mistake to see it as a sport that is only defined by your physiological abilities, which it just isn't.