I think it's more about not wanting to port the steam client to arm than it is about porting the game. There's source engine games ported to ppc and arm platforms after all.

> I play Dota from time to time on my M1 Max Macbook Pro and it runs more than fine.

Dota on macOS doesn't ship an arm binary and uses moltenVK to translate graphics api calls, it's on borrowed time if valve doesn't want to ship proper binaries as rosetta isn't gonna exist in a few years.