This is just a question of semantics. If valve wanted to commit to anticompetitive practices they easily could (e.g. make it so that games published on steam can’t be published on other desktop storefronts as well, I think most publishers would be forced to stick with Steam). The only difference between our arguments is that what you call a “monopoly” is a company that actually commits to and controls the market with such practices and what I call a “monopoly” is a company that has enough market power to do so. (And I think both are totally sensible definitions of the word “monopoly”)