If you repeat the question again, the answer will still be, that's not how it works. Me explaining that mistake is the reply. I am only "going on and on" because you continue demanding this explanation, and then, you ignore it.
The sentence you quoted contains the words "what we do about it" followed by what we do about it, and what we do about it is, not much. It is not the sort of problem you necessarily do something about. The reason for saying so anyway is cleverly hidden at the start of the quoted sentence.
It is still a problem. It is still something we have to stay aware of, to prevent complacency... or whatever else you'd call it when simply saying "Steam is a monopoly" unleashes a torrent of denials and accusations, as if the fact must be followed by a call to action obliterating everybody's favorite DRM vendor.
Steam has overwhelming market share, and could abuse that in a heartbeat, in a way that no merely competitive company can. Steam is so large that not being on it, for whatever reason, can be a death knell for sales. They wield such power that their cautious selectivity dictates which games most customers would even see offered - and an accept-all policy would dilute the shopping experience for nearly everyone.
All of that is real even if there's not a safe and satisfying alternative, immediately to-hand. Unless you've got a magic wand laying around, there's nothing I'd ask you to do about this situation, besides maybe could you stop asking me what to do about it? It's a systemic risk. That doesn't mean it's an emergency.