That "rule" doesn't apply to anyone. Google is not in trouble for making their own operating system and store and using them on their own phone. They are getting in trouble for the way they try to get other phones to include Google's store.

With Apple the OS maker, the store maker, and the phone maker are all the same entity so it doesn't even make sense to say that the OS vendor is trying to force their store on the phone vendor.

If Apple draws antitrust scrutiny, it will be for something different that what is drawing attention to Google, such as for not letting third party stores on their phone.