Part of Epic's argument was that they wanted Apple and Google to distribute Epic Games Store, which makes your argument valid. However, the commenter you were replying to likely isn't aware of this and does not care about how they install EGS, only that they are able to do so if they do not want to deal with Apple or Google.

The sneaky part of Apple's counterargument is that they try to apply your argument not to their own App Store, but to the entire OS. This is a land grab - the App Store is their service, but I own the phone, so the OS should respect my decision to circumvent Apple's curation if I so choose. If I'm not allowed to sell in Target or Walmart, I'm allowed to open my own store and people can circumvent Target to go to me directly. This is how it works for every other form of computer except phones and tablets.