'Some people insist it's not a monopoly' is restating the problem.

Any discussion of Steam's overwhelming market share requires the question 'why does overwhelming market share matter?' and the answer is 'because that's a monopoly.' We could dance around the word itself, and just describe an elephant, but that only delays when people with misconceptions hear the word for this animal and go 'nuh-uh.'

That delay would be worse - because it allows for people who acknowledge all the reasons Steam is a monopoly, and all the reasons that monopoly is a systemic risk, but will still insist with kneejerk certainty that Steam doesn't count. It's some other distinct thing, despite the absence of difference. That denialism is a lot harder to deal with than just telling people:

Real markets never have absolute monopolies.

When people say monopoly, they mean the legal definition.

Dictionaries which do not reflect that use are incomplete.