I don't think it's a bullshit smear, she said this in 1990 (they didn't divorce until 1992) and they followed up on it with interviewing him
1 September 1990 vanityfair.com/magazine/2015/07/donald-ivana-trump-divorce-prenup-marie-brenner
Donald Trump appears to take aspects of his German background seriously. John Walter works for the Trump Organization, and when he visits Donald in his office, Ivana told a friend, he clicks his heels and says, “Heil Hitler,” possibly as a family joke.
Last April, perhaps in a surge of Czech nationalism, Ivana Trump told her lawyer Michael Kennedy that from time to time her husband reads a book of Hitler’s collected speeches, My New Order, which he keeps in a cabinet by his bed. Kennedy now guards a copy of My New Order in a closet at his office, as if it were a grenade. Hitler’s speeches, from his earliest days up through the Phony War of 1939, reveal his extraordinary ability as a master propagandist.
“Did your cousin John give you the Hitler speeches?” I asked Trump.
Trump hesitated. “Who told you that?”
“I don’t remember,” I said.
“Actually, it was my friend Marty Davis from Paramount who gave me a copy of Mein Kampf, and he’s a Jew.” (“I did give him a book about Hitler,” Marty Davis said. “But it was My New Order, Hitler’s speeches, not Mein Kampf. I thought he would find it interesting. I am his friend, but I’m not Jewish.”)
Later, Trump returned to this subject. “If I had these speeches, and I am not saying that I do, I would never read them.”