the power to charge overly high prices

One doesn't have to actually use a power in order to have that power. If I was carrying a loaded shotgun, I would have firepower. I wouldn't have to actually fire the gun to have firepower.

Also, one could argue (and Epic Games has) that Steam's 30% cut is overly high for digital distribution. I'm not sure whether that's true or not, but that doesn't really matter to the question of whether Steam has dominant market power.