There's another reason they've been encouraging devs to use Proton on Linux instead of a native build: you're less likely to ship Linux-only bugs that can't be replicated on Windows, which saves a ton of debugging (and maintaining a separate dev environment) to fix something a minority of players will ever encounter.

And those Linux-only bugs that do pop up are likely an issue with Proton itself, and Valve directly benefits when bugs in their product are reported.