That's the old Debian-based Steam OS 2. The Steam Deck uses Steam OS 3.0 (Arch-based) which is not yet officially released.

In the meantime, there's HoloISO [0], but it's unofficial.

[0]: https://github.com/HoloISO/holoiso

This isn't the same SteamOS as the one that the Deck is running, it's an old one from their Steam Machines. Valve should take that page down or put up some sort of warning as it's been confusing people.