FWIW, it's clearly different: the GPL can be used for manuals, but is not an FDL-compatible licence, because it does not allow addition of unremovable adverts and the other FDL problems.
These are not problems, again you spread more FUD.
So, requiring FDL-compatibility means requiring something less sharing-protective for the manuals than the GPL. :-/
The GFDL is as sharing protective as the GPL, but it it is meant for documents.
Yet again you start spreading FUD about the GFDL, please stop. If you have a opinion, state it as that, but to this point it has always been FUD.