People keep saying that and it still isn't true. It was argued the dictionary definition wasn't good enough, so how about US legal criteria to be considered a monopoly.

Federal criteria looks at three things: market power, exclusionary conduct, and business justification.

1. Market power: "...significant and durable market power — that is, the long term ability to raise price or exclude competitors." You could argue this as I believe Steam has something like 75% of PC game sales. However I don't think it would be difficult to argue that console gaming is also a market competitor which would bring their market share down some, but I'm not sure by how much. There is no hard limit for what market share is required for the firm to match this criteria. "if competitive forces or the entry of new firms could discipline the conduct of the leading firm, courts are unlikely to find that the firm has lasting market power." It is likely that if steam began being anti consumer, the existing competitors could take advantage, or other markets could be developed. So this criteria is in the maybe column.
2. Exclusionary Conduct: "Obtaining a monopoly by superior products, innovation, or business acumen is legal." This alone means that even if they have the market share to be investigated as a monopoly, they haven't broken the law since they allow open competition against them. They have simply provided a superior product to consumers. "Exclusionary or predatory acts may include such things as exclusive supply or purchase agreements; tying; predatory pricing; or refusal to deal." I am unaware of Steam doing any of this. Epic on the other hand, absolutely does.
3. Business Justification: "The monopolist may be competing on the merits in a way that benefits consumers through greater efficiency or a unique set of products or services." Steam is providing a unique set of services that benefit the consumer. So it seems that have a business justification.

Put simply, Steam is not a monopoly by the dictionary definition and likely wouldn't be found to be a monopoly in a legal sense either.