That's irrelevant, because Proton is literally a Windows emulation layer, (the product of decades of cumulative work). "they" (Larian) didn't have to do anything for that.
Certainly Larian's effort for making a Steam Deck native version is commendable (I hear it was the result of one single employee's effort). Larian is a rare beacon in the video games industry biz for the amount of post-launch support and content they provide.
But my point remains: supporting Linux broadly is a far larger, and ultimately unreasonable, ask, than just supporting the Steam Deck.