That's still Steam bringing games to Linux though, right?

Whether or not it's native or wine doesn't change the fact that Steam is pushing into Linux. They've shifted the responsibility from pushing linux support on the developer to providing linux support themselves.

Linux needs to attract gamers before game devs will widely support Linux. It's a chicken and egg problem that proton neatly sidesteps.

Valve will have a much easier time improving proton than they will forcing devs to make Linux compatible games for little monetary reward.

Windows as a library, what's not to love? This gamer cares whether the games run, not about nativity.

In order to attract more developers to develop on Linux or at least make it work through Proton, Valve is taking the path of least resistance. Proton seems to be a good step toward that end.