"way WAY bigger" is a major overstatement. But even if that were true, SteamOS can handle lots of other non-steam storefronts.

For example, I recently went and played some World of Warcraft for old times sake with some friends on Linux. All I had to do was open Lutris which is available through a Flatpak on the SteamOS discovery tool, and then from Lutris you install the Blizzard Launcher. Opening the Blizzard launcher lets one install and play any of their games.