He was also a Communist more than a Hitler Apologist.
That is Ann's point. He supported Hitler when it was the "party line" to do so. Calling someone a Communist is considered a compliment now, so Ann stressed the CP's pro-Hitler position during the Molotov-Ribbentrop pact.
Ironically, Trumbo also wrote the screenplay for Exodus. The Communist world internationally had turned against Israel by then (though it hadn't yet broken relations), but inside the United States the hard Left continued (doubtless on Moscow's orders) to claim a monopoly on pro-Israel sentiment.