Zelenskyy's got nothing to lose now that Trump has all but admitted he's carving up Ukraine and doling it out to the highest bidders. But he's also not long for this administration. He's outright said he won't run for another team (assuming he could even get an honest ballot). Trump's going to force in his own MUGA replacement with the next election.
If Zelenskyy doesn't run for another term wouldn't his endorsement (or... unendorsement???) make or break the election? I'd think Ukraine would be safe from MUGA for a few elections.
I think he just has no concept of what is and isn't true. His experience is that the determining factor of success is shouting and punishment, nothing to do with truth.
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.