Please don’t use that guide. That’s the old 2.x line of SteamOS which hasn’t been updated since 2019 and is based on a very old version of Debian. The SteamOS that the deck uses is 3.x and has also moved from Debian to Arch. Valve really needs to update that page.
If you want to build your own SteamOS machine, take a look at something like HoloISO (github.com/HoloISO/holoiso) which is built on the current branch of SteamOS with the deck-exclusive OS/hardware items swapped out for standard kit.