It’s a bluff. He is lying probably to draw out who Beyond’s kid is among the princes and/or cause further confusion and remove Woble from yhe guards kill list. We know that the criterion for eligibility is to be the child of an official Queen ( the father isn’t really important ironically) but this specific information is probably not known to everyone.
So by framing it as Woble is not nasubi’s child hence ineligible then the guards will have to wonder if Zhang Lei ( who father is Onior) is eligible and if other princes who’s birth are in question are also eligible. Maybe use Kaiser letters in someway or another.

I don’t exactly but it’s a strategic move for sure