Do you have a source for this? Im confused by this, as the GPL section 7 is pretty clear that additional restrictions are effectively void. I suppose it’s technically not contrary to the GPL to idly state those restrictions, but it is contrary to the GPL to expect them to do anything. If the author is deliberately including an impotent clause in the hope that people will follow it anyways, I feel that trying to confuse or scare people into doing something the GPL gives them explicit permission to do is contrary to the spirit of the GPL.

Furthermore, trying to retaliate against people who (as permitted by the GPL) remove the citation notice, as the author here has done, seems very contrary to the spirit of the GPL.