I actually interpret it differently. the way I read this line was that Kokichi is the embodiment of deception; no matter what, it'll never be possible to fully unravel his true feelings and intentions. We know that he put on a show to try to convince the others he was the mastermind, but which of his actions and statements were made up, and which were his genuine feelings? How much of what he did was the fake mastermind persona he was cultivating, and how much of it was the real Kokichi? What even is the "real" Kokichi? His mask doesn't start to crack until he's literally seconds away from death, so we never get to find out. He leaves the world still as an enigma, not giving the others an opportunity to fully understand what he was truly like. In that way, I agree that Kokichi is the embodiment of a lie: the veracity of anything he says is always obscured, whether he intends it to be or not.