The responsive template and the old templates have always had an incompatible URL scheme (even before Twig); this is on of the reasons why there are issues describing broken URLs.

Routers are a new component that tries to to find a suitable URL with the given object or string; in this case you feed it the FileDescriptor and the Router should return a correct URL.

Disclaimer: I do not know whether the File Descriptor has routing rules defined; it could be that those are not implemented because I didn't encounter it yet