Yes, Mono is used by Wine to support Windows .NET applications since it's a) open source and b) contains support for Windows Forms and other Windows-only APIs.
They can't ship the regular .NET framework by default for licensing reasons but it can be installed with winetricks to replace Mono, which is sometimes necessary for compatibility reasons.
But it does, paying third parties to not publish on your competitors platform is the oldest anti-competitive behaviour in the book.
It would have been completely fine if they started out with actually funding development of new games and only releasing them on their store.
I would have even given them some slack for their bad launcher since they were new to this.
Instead we are here, almost 6 years later. Their launcher is still trash, their exclusive deals were a complete money sink, EGS is still not profitable, they burned all bridges to Valve and are not one step closer to their claim that 30% is too much and they can do it with ~~8%~~ 12%.