Sounds funny if you consider that Whatsapp itself is just a branded deployment of FOSS XMPP server Ejabberd, with feature of federation taken away. Plus a client app implementation, of course.
Nothing in the AGPL prevents either from occurring.
No idea whether the assumption is true that Whatsapp is just a branded deployment.
As the countless messaging apps out there demonstrate, the value of communication apps lies in the network they build. If they provide a decent UX, and successfully bootstrap a network large enough, it wont matter if their code is open.