I thought it would be because he also ghostwrote the Star Wars movie novelization, which was credited to George Lucas, and wrote pretty much every sci-fi movie novelization in the 1970s and 80s.
Now that I think about it...yeah, he was credited with the TMP story.
Yes, the news was never supposed to break that he ghostwrote "Star Wars" for Lucas. But the incorrect attribution to the first French edition of the TMP novelization had people connecting the dots.