This get brought up often, and it’s such a lazy example. Apple forces you to give them their cut (you have to pay for developer license even just to keep your own personal apps on your phone).
Valve's most anticompetitive rule is that steam keys you distribute outside steam shouldn't be sold for less than the price on steam. Would that Apple were the same.
I cannot get apps on my iPhone from anywhere else but the App Store. While they are dominant, Valve isn't locking anyone in even on their own hardware.
Valve isn’t forcing you to use Steam.