Maybe it should be something like, "if your platform doesn't allow side loading of other apps, or other app stores, then the store on that platform can take no more than X% commission on sales"
PC has the Microsoft Store, Mac has the App Store, and Linux has it's own distro repositories depending on flavor.
SteamOS is just a tweaked Debian distribution, and you have full control of the os, so you can install any competing software you want.
There is a menu item labeled 'Add A Non-Steam Game To My Library', after adding you can also use steam's compatibility tools (Proton, or DOSBox or whatever custom compatibility tools) with the games as well.