Proton is just Wine from Valve. They add their own fixes and patches and whatnot and have an “experimental” branch you can try with games that don’t work right away, but it’s just Wine. Everything Valve does to Proton eventually makes it way back upstream to Wine proper. One reason Valve may not make it available for MacOS themselves is because they’re basing their SteamOS on Linux, and while MacOS and Linux are both Unix “like”, MacOS was/is more based on BSD, so the system calls may not always line up or work exactly the same when translating them. I do think however that Proton, or a modified version of it at least, is what Apple’s game development kit thingy leverages.