The blog post says they will focus on making controller play better on Switch, then make that generic for PC(which implies Steam Deck as well). The PC/Steam Deck version supports mods, which is better than Switch anyway.
Regarding the ecosystem, the Steam Deck is a x86 PC. It comes with SteamOS, which is an open Arch Linux based distribution. You are free to install other operating systems like Windows on it. The out of the box Steam experience is quite good, with lots of games on the Verified list[0], and they play really well. People also install non-Steam games and software, including console emulators, and there are growing communities around that.
Elden Ring plays well. I don't plan on using my switch OLED until the next Zelda comes out.