Interesting study, bad subject
I think Trump is not a good liar, nor does he care to be. When he makes statements which are factually, provably wrong, he doesn't really try to justify them. He has said he views these statements as "truthful hyperbole", though there is nothing truthful about them. Trump lies to intimidate, confuse, or (most often) to dog-whistle his base. He doesn't intend deep deception. Nor is he smart enough to carry it off.
It would be a tougher test of the lexical analysis if the researchers used it on statements by Putin, who is, I think, a masterful liar. Putin does lie to deceive. He does it with a chess-players long view of tactics and strategy, and with the smiling lubricity of a master poker player.
Where Trump flaps and squawks, Putin smiles and slithers.
Incidentally: the "all politicians do it" line doesn't work. No politician on either side of the Pond lies nearly as often as Trump lies. He is really exceptional in that regard. It's like comparing a guy who habitually drives on the sidewalks, runs over pedestrians, and smashes into other cars with someone who has gotten three tickets for running stop signs.