Just got my 512 GB Machine today. Its hostname is steamdeck and the default Linux user is deck. I would have expected them to at least change the hostname in SteamOS. No wonder all the Deck ready games use a Steam Deck profile if not even Valve bothered to rename the thing.
Wonder if it also sets the steamdeck environment variable.
Can change it if you want: Settings > System, and scroll down to ‘About’, change the name and restart.
Seems like the problem is maybe Steam running with SteamDeck=1?