Idk if admins need to manually approve registration applications for Mbin. Many smaller Lemmy instances are set up that way, so it may also be the case for Mbin.
Regardless, whether the registration is reviewed manually or automatically, the admin is the one who will be able to help you.
It doesn’t need to be the answer. It just needs to be an answer for certain use cases. Both platforms can easily coexist. That’s the beauty of federation.