If anticompetitive means "it's your choice to enter into an agreement in which we host your game for 30%, and distribute it on our platform, with unlimited patch updates, and unlimited user downloads, and a fuckton of features like community forums, guides, groups etc., also if your game is good we will promote it free of charge"

Then I suppose companies like Epic who choose to run at a loss, as opposed to providing a good service, have no chance, and Steam is anticompetitive.

The counter narrative exists though, Steam is just a good service, and if you want to compete with them, you need to provide a good service, like GOG.