Looking at steam’s own policies, this is true for steam keys, but there is an an going lawsuit that claims steam also makes this apply to non steam-enabled games: arstechnica.com/…/valve-issues-scathing-reply-ove…
But looking mosre closely than I did previously this is based on:
An contract that is apparently not public
A 1 time example that Valve denies
So I don’t really know, but if what valve says is true (which looks like it is), then I don’t see any monopoly abuse indeed.
They do have a monopoly, but it’s in large part for providing a better service. As a Linux user, I prefer Valve 100% over Epic that buys Rocket league and discontinues linux support. I do prefer Itch and GOG for the possibility of no-DRM games, but I’ve got to say it’s overall a worse experience (no auto updates, no social features etc…)
I made my initial comment after watching: www.youtube.com/watch?v=mOEG5qmMQas which suggested that Steam applied the MFN for non steam - enabled games too, but was done prior to Valve’s response.