I explained more details in the previous blog post (https://kokada.dev/blog/from-gaming-rig-to-personal-computer...), but my current setup replicates a SteamOS experience in NixOS using the Jovian-NixOS project. So basically the same thing, the system boots in gaming mode by default and I can switch to the KDE based desktop by select "Switch to Desktop" in Steam's Big Picture mode.

So no, I am not using KDE for gaming itself (keep in mind that it is possible, the games runs well under KDE too), I am using KDE in my gaming rig (that is, my desktop that is focused in gaming), but for gaming itself I generally use Steam's Big Picture mode.