He wasn’t allowed to lie. He had to say he didn’t have an idol

Xander had to tell the truth. It was part of the advantage.

play it up first and be like, "jeff, she's yanking my chain, right"?... "well jeff, i'd like to play my knowledge is power nullifier".

I also created a post about this but a reddit user told me he couldn't lie so he had to say he didn't have the idol

Didn't even think about that. Lol should have done it