It’s not too difficult to add support for Linux. Steam already includes a set of libraries that add binary compatibility to Windows games on Linux, called Proton. If I enable Proton manually in Steam settings, some games will work, some others will not work, and yet others will require some tweaks.
If a developer tests their game using Proton on a supported Linux distribution, which is likely just Ubuntu, a checkbox can be ticked for the Steam release, and the game is then going to be tagged as having native Linux support. It doesn’t need to be recompiled for Linux with exotic libraries or anything, just the exact same Windows binary needs to work reasonably well on Linux/Steam/Proton without tweaks.
Something else to consider: if the game works on Steam/Proton/Linux, and it can be played using the input devices available on a Steam Deck, it can end up as a supported title on that platform. That may end up justifying the effort.