The law does not require that Google and Apple run their online stores the same way, or that Apple interact with Google's store the same way they choose to run their own store.
That's correct. But the law does require them to run the stores in a way that does not violate antitrust laws. Apple is violating antitrust laws by forcing companies to use Apple Pay as a condition of using the App Store (i.e., leveraging dominant/monopolistic market position in one market in a non-competitive manner over another market). Apple cannot then do the same thing it prohibits in its own app store in the Google Play Store, which has similar rules, because both of these actions have the effect of taking legal positions.
In fact it's arguable that a free marketplace encourages and depends on different companies taking different approaches to meet customer demands.
Which Apple does not permit. It's literally Apple's way, plus a 30% cut for doing nothing, or you're not even allowed to play. And that's exactly what antitrust laws were intended to prevent.