Comment by OskarS

That is how I (a non-lawyer) understand it as well, but I wonder if it's so simple when you combine it with the GPLness of it all. Like, releasing something under the (A)GPL is a license to use and modify the code how you see fit, and that goes "virally" through the forks. This fork is just using their own GPL-licensed code, and it seems unreasonable (for some definition of "unreasonable") to limit forks in this way. I think it's plausible you can make an argument that if you make this kind of restriction in your GPL codebase, you're violating the GPL license of the original ("upstream") authors.