The key here is simply NOT to get into useless arguments with people who are not interested in listening - and especially not at airports where you’ve gone past the checkpoints ... :-) ...
Right, arguing with them can backfire and make you look bad.
That’s why it’s better to quietly get a flight attendant’s attention, maybe while you are near the restrooms, and say you are worried because you heard that Muslim passenger in seat 39A talking on his cell phone before takeoff about “jihad” in Arabic.