The phones are simply Android phones with different app store with different policies.
Users can install from Google if they still want.
If fact they can rebrand another phone and add another appstore.
All they have to do is to rebrand one or more existing phones and change the app store policy. Only the app store software needs to be changed.
What is so difficult about that?
Five years is more than enough time to establish a new more liberal ecosystem for app developers if Google and Apple won't play ball, so what is holding them back?
Huawei, Redmi, Samsung and Amazon have their own app stores, so why should it be difficult for Epic et al to have theirs on their own phone as well?
What so difficult about that?