...I wonder if #GenshinImpact _now_ being playable over GeFN is related to this.

#GenshinImpact seems to be running on Linux right now, it's likely due to someone in the dev team breaking the anti cheat (again) as no official statement has been made about #SteamDeck support but maybe that's happening. At this stage keep an eye on it https://www.reddit.com/r/linux_gaming/comments/123zllz/genshin_on_linux_without_custom_code_now_works/