You could also think that, because of WINE, Windows is first class and then they check Linux with Wine.
Depends on the perspective.
PS. Yes, steam detects Wine
By making Linux compatible with Windows games it gets rid of that objection "I'd move to Linux if it weren't for my games" which was the remaining objection for a LOT of people.
Because Steam tracks WINE that's a very good thing, so they can detect players who bought their games to play it on Linux.
This helps encourage Linux native gaming growth, because developers can see the chunk of Linux players rising as more get rid of Windows because they no longer have that remaining obstacle.
So, yeah... maybe that means most devs will just say "my Linux support is just it runs on Proton probably, good luck!" but the thing is... there are games on Linux now. Lots of them. Lots of good ones. I was playing The Witness last night by just pushing play on Steam. No winecfg or winetools or separate DriveC Steam installation. No messing with drivers. I pressed play, the game loaded, I played it. I've repeated this loop in the last few months with most of the games in my library. Endless Legend is back in my rotation again. All of the dumb anime Japanese games where they don't even know Linux is a thing that exists suddenly work. It's glorious.
Wine/Proton may be the lazy way to develop for Linux and might not give people the coveted title of Linux exclusive gamer, but it's working really well if all you care about is playing games and not installing Windows.
Also the "no tux no bucks" philosophy many Linux users take in avoiding paying for non-native games.
Contrary to FOSS folks, professional game studios aren't religious about APIs, as long as there is money to be made.