Google can vet the app store but they are not going to be able to vet every app that’s inside that app store.
I’m not completely clear on whether Google will just be forced to allow alternate app stores into Google Play or allow more apps into Google Play. Either way I’m not sure I see the point of an app store inside an app store. We can already install whatever app stores we want.
There’s also the existing problem of app stores like the Samsung app store updating apps that weren’t downloaded from their store, which seems to me would open things to security vulnerabilities if one of the front ends gets hacked and then starts updating apps that you didn’t even download from that app store.