the first paragraph has a link to a protondb dashboard:

https://www.protondb.com/dashboard

pretty cool visualisation, though it's not obvious where they get the 90% for me either. But for example if you take the Top 1000 games, 89% of them either have a plat, gold or silver medal, which sounds quite equivalent to "works on Linux". There is an "all steam games" option, but the issue is that most games haven't had any feedback from anyone on protondb

edit: the stat comes from the second link https://boilingsteam.com/windows-games-compatibility-on-linux-is-at-a-all-time-high/ which uses as a sample all of the steam games rated on protondb

Iirc correctly, those are just the Linux native ones. A game doesn't have to be native to play just fine on Linux though.