Kinda misleading.
First of all, games do not have to be on Steam & launch through Proton to be able to run on Linux. Wine hs gotten extremely good too, even if it may require a bit more tinkering in comparison.
It's also not like this because of the Deck. Proton has been on a good run for several years now, which was very much evident based on the stats on ProtonDB. The Deck helped more with popularity & spread of Linux, rather than actual compatibility.
And those unsupported titles are almost all competitive multiplayer games. Regular multiplayer titles that are mostly PvE focused work usually fine under Linux.