The core point your parent was trying to make is that Fortnite is not an open market. No one but Epic provides goods to the Epic store. Apple allowing third party developers is the distinction being made.
Apple's platforms aren't an open market either. They allow third party developers to develop for their platforms, subject to you agreeing to their terms.
Agreeing to a TOS is a standard for open markets. Splitting hairs over a TOS existing is ignoring the fact that Epic doesn't allow third parties to sell in their market (with or without a TOS).
When you accept Apple's developer agreement, you're agreeing to a contract. It absolutely isn't an "open market" for any definition of what that means. Apple has exclusive control of their IP and they are offering a way for you to license it on their terms.
Regardless of what term you use it's a fact of reality that right now no one else but Epic can put content on the Epic Store which is not true of Apple and the App Store.
Which would be irrelevant since the original comment was that Epic had no monopoly on any market, and didn't specify the terms and conditions for a market to fit the argument.