Aero bikes make only marginal gains whereas the rider accounts for the most drag, it's common sense. But those marginal gains are important to those racing against the clock where every second counts for them - unlike most cyclists who couldn't care less about a handful of seconds or even a few minutes, here or there.

On the flatter routes I do and for sprinting my aero bike is noticeably and measurably faster for me to use - same engine, me, as the constant, so yes, times can be measured against my non-aero bike to prove it. Of course, aero can only go so far and the steeper the gradient, the less the time saving. My much lighter 'climbing' bike is night and day faster than my heavier aero bike on steeper gradients - that's not just how it feels, I have seen the time differences across the segments on Strava for the same watt efforts.

So sure, the rider makes up the vast majority of the drag which accounts for why rider weight, shape and body position is so very important along with technique, the ability to spin and power. The bike, however, does contribute and can make a difference to those racers - and you do not need to be a Pro to realise them - who want every advantage. Obviously too, a rider who is appreciably stronger, quicker, than another will still be faster overall even if riding a much heavier, less aero machine. The proof of difference a bike makes must be calculated by the same engine - rider - on different bikes at the same watt power over the same course in the same conditions to ascertain those time differences. Being in the top 5 in my region/state for my age category, I'm fast on my 29er XC MTB and can beat most regular Club-level road cyclists in my area on it but that's because I am the stronger cyclist and can beat them riding , what is essentially, a brick with wheels. That doesn't make my MTB the equal of my aero road bike. I'm far faster on my aero road bike.