I think they calculate proton to be part of the Steam client(with which I agree)

It's not, it's a separate component downloaded with the game.

while the 32 bit trickery of the windows api did come build in with Windows

I'm not sure what you're meaning. Even if it was necessary on Windows, doesn't mean it's necessary on Linux. Aren't Proton games even run in a container (pressure-vessel), so the host system doesn't matter that much.