That "political motivation" is what makes his statement suspect. He'd say that about anyone who didn't fit his "ideal" of what a Mandalorian "should" be. This would likely include any who still follow the old ways. It's the same as saying someone is not a "true" American or a "true" Fan, because that person doesn't follow the same philosophy as you. If you want to distance yourself, and your people from their warrior past, it makes political sense to claim that someone who still follows that path does not belong to your people. That inherently makes his statement suspect. He is an unreliable witness.
By contrast, Watto has nothing to gain by attributing his immunity to Qui Gon's Mind Tricks to his species. If he were to lie, it would make more sense to claim that it was his own ability that kept Qui Gon from influencing him.
Edited by Tramp Graphics