It’s possible that he never discussed any of that with Howard when they were planning the movie. Vance wasn’t heavily involved in the process, he didn’t write the screenplay or produce it. They probably just had a few discussions about how the book should be translated into film, as a courtesy. And Vance at that time was very anti-Trump, so if they did discuss politics in passing that might have colored Howard’s views on him. None of that means Howard is “full of shit”.
I haven’t read the book or seen the film, so I can’t speak to anything specific on the adaptation, but according the the review linked in the article Howard did little to change the overall message, so he deserves criticism on that point.