While I see DXVK was important, Valve's history with Linux is much older. I would place "anime girl thighs" on the second domino
SteamOS was first released in 2013, just before they released there first hardware running Linux, the duly forgotten Steam Machines in 2015.
I guess they don't disclose it because they change it frequently. If whatever new arm chip became discounted, they just switch to that.
I noticed this a long time ago, Asus did this with their entry level routers frequently, but they at least disclosed it. You had to be careful, the same router with the same name could be totally different inside, only the revision number changed.