I hate to agree with a billionaire, but he's right. Currently steam is not a monopoly at all, and hasn't yet gotten so dominant that it amounts to one.
But they are the single most dominant outlet, and are in the first stages of having an ability to control the entire market in unfair ways to the point of crushing competition. It won't be long before they do have that ability completely. And we can't just trust the billionaire because he's shown an unusual degree of user focused choices overall. Even if he's perfect, he ain't immortal, and as soon as he's not in control, there will be fuckery