Yeah, calling Steam a monopoly is a disservice to what we really mean when we say something is a monopoly, while they do control the market share by a large margin, they do not employ exclusive control or exclude competitors (if anything Epic attempts to do that, though it seems they have calmed down a bit).
You could call them a natural monopoly, but even that isn't really useful in what we think of for others of that type. Steam has plenty of competitors and doesn't seem to engage in monopolistic practices generally, the market just vastly prefers them.