From that FAQ: "If someone asks you to sign an NDA for receiving GPL-covered software copyrighted by the FSF, please inform us immediately by writing to license-violation@fsf.org."
Also from that FAQ: "If the violation involves GPL-covered code that has some other copyright holder, please inform that copyright holder, just as you would for any other kind of violation of the GPL."
Right.... So it's probably perfectly binding. If they hold the copyright outright, they can create such conundrums lik e conflicting licenses.
If it incorporates the work of others though , it's tricky.