Since anyone is free to register anywhere ..No, anyone is free to create his own pod and eventually change the code on it if he wishes to.
network fragmentationI guess network fragmentation isn't inherently bad and maybe the only solution to moderation/censorship by algorithms in big networks.
Pod’s can close registry. Pod’s can enable invite only, pod’s can kick people out, pod’s can ban profiles from other pod’s and if they can’t right now eventually block entirely other pods.Yes, of course. However as a whole the system is pretty much unrestricted. A lot like e-mail, only with a little more verification. This ensures diversity and helps freedom but it also enables misuse.