Linux is strategically important for Valve to maintain a credible alternative to Microsoft. Note they don't even have to necessarily ever "pull the trigger" to force Microsoft to make sure they don't lock them out, simply having the credible option to do so is sufficient to achieve their business goals.

And having picked up a Steam Deck about a year ago now, going into it with the expectation that it would run maybe half my games and tending away from the AAA releases, I can say they have a very credible option. Valve can run almost everything without Microsoft now. Pretty much everything I have runs on my Deck just fine, since I don't do the multiplayer games that have aggressive anticheat on them. I almost don't even think about compatibility any more because it's just there, for what I do. This has spilled over to my Linux desktop, too; I switched it due to having my fill of Microsoft's belief they own my system, own my users, and can do whatever they like with them, and if that cut off all gaming it would have been fine with me, but in reality the spillover of Valve's efforts is that it works just fine.

Apple has chosen to make themselves strategically unimportant to Valve by making it probably even more difficult to support them well than to just give up and submit to Microsoft entirely. They could have played this role but have chosen not to, because in their world, this is relatively minor compared to what they want out of their platform.

So even if the user count is roughly the same, Valve is going to support Linux much more, and is pushing it harder.