Why, that's exactly what it is? It's a named pipe, but not one used explicitly by you but implicitly by the process substitution feature to pass a file, which of course has a name, to another process. It's only anonymous to you because you didn't name it, to the system it's an ordinary named pipe.

Anonymous in that their name doesn't matter and you need not care about it. They are implemented using pipes with actual names, but the names aren't important from the shell user's point of view.