Trump's thing is being completely uninhibited. He doesn't have that little voice in his head saying "no that would be stupid" or "you will face consequences if you grab the cop's service weapon." He doesn't experience regret, shame or anxiety, because he's at best vaguely aware of the past and future.

That's why he's not precisely a liar. He simply doesn't understand "truth" and "falsehood" as meaningful categories. There's only his emotional state at that specific moment, that's what defines his reality. If he gets a thought, he'll voice it immediately. If he experiences an impulse, he'll act on it, no matter what.

That's not exactly a good thing, but it does make him pretty unique.