Indeed, even much earlier. With Steam Deck they achieved wider adoption but the first generation of Steam Machines came out in 2015 and they have been committed to the SteamOS linux distro since then.
Yeah, I'm sure you're right overall, they've been at it for a long time. I think it's worth keeping in mind that all of the SteamOS'es before Steam Deck were pretty much nothing like the current (3.0) iteration. If I recall correctly, I think they were based on Ubuntu or Debian, compared to the current Arch Linux distribution.