There is absolutely nothing to back this assertion up. So you base the whole of gamers on what your son does, do you?
There certainly is something to back this assertion up. It's called logic.
If Elite: Dangerous was initially released on Steam and only available through Steam then the Steam numbers could be counted as being very accurate. Elite: Dangerous was initially released without steam support. Now ... if I wanted to take the time to get a Steam key for Elite and register the game in Steam ... why would I do that if Elite: Dangerous is the only game I want to play? It would be counter intuitive. Logically I would only be interested in linking Elite: Dangerous to Steam would be because I already use Steam for other games.
Therefore Steam users are more likely to be people who play multiple games. Many players who purchased the game early, or backed on kickstarter, did so because we played the original games in the 80s or 90s.
Logically: If you play lots of different video games then Steam is an attractive storefront and platform for your gaming needs.
Logically: If you only play one or two different games and you purchased those games directly from the developer then Steam doesn't really provide you with any benefit.
Personally I play Elite: Dangerous and Kerbal Space Program. Purchased both directly from the developers. Steam is an unnecessary background process.