https://store.steampowered.com/steamos/ says

So, what is SteamOS?

SteamOS is a public release of our Linux-based operating system. The base system draws from Debian 8, code named Debian Jessie. Our work builds on top of the solid Debian core and optimizes it for a living room experience. Most of all, it is an open Linux platform that leaves you in full control. You can take charge of your system and install new software or content as you want.

I guess it's a bit confusing because their Steam Runtime was based on Ubuntu.

SteamOS was indeed based on Debian, and Ubuntu was the officially supported distro of the Steam desktop client, and now the new SteamOS is apparently Arch based. Distro-hoppin' like a Boss! ;)