Definitional arguments are the least entertaining for anyone. So I'm sorry we have different definitions. Allow me to expand your quote from Wikipedia
"In law, a monopoly is a business entity that has significant market power, that is, the power to charge overly high prices, which is associated with a decrease in social surplus.[3] Although monopolies may be big businesses, size is not a characteristic of a monopoly. A small business may still have the power to raise prices in a small industry (or market).[3]"
In your opinion, can steam unilaterally raise the price of all games, and people will just have to accept it? If yes then sure it's Monopoly.
If steam increases all game prices by 100%, and doesn't give the developers a cut, I imagine we're going to see people move to GOG epic etc.
Update: https://www.cheapshark.com/search#q:The%20Last%20of%20Us%20Part%20I
I've never heard of this site before, but it compares prices against different game stores. So last of us part I, is available on a variety of different game stores, at different price points.