The joke is that he can do this, but once only as afterwards he will no longer have the power to do so.

Realistically it will just be passed anyway one way or the other. e.g. the UK will become a republic, or they will bypass the monarch like they did in Belgium when the king refused to sign the abortion law (parliament declared the king "incapacitated" in which case the PM could sign things in to law, and a few hours later they declared him "capable" again – whether this really was keeping with the spirit or even text of the law is dubious, it's not in my reading, but no one really cared; the only reason Belgium still has a monarchy is because the king didn't object to this hack).