I follow you, but disagree. If P is meaningless, we can't say anything useful about it - even "!(P&!P)" doesn't necessarily hold!

As for your specific argument, it begs the question. If you assume that a sufficiently smart AI could always convince someone it lacks P, then you've already assumed the premise (a sufficiently smart person can detect P) to be false. Of course you may find the assumption reasonable, based on your intuition about what P means, but someone who disagrees would just reject the assumption.