I haven't read it and it's possible that Wikipedia is softballing it, but it seems like it has a series of segments about Mohammed's life, including a questionably true bit about him adding and then later retracting three verses from the Quran (the eponymous "Satanic Verses", since allegedly the reason they were stricken was because Mohammed realized they were told to him by Satan instead of God). Aside from that, it uses a lot of outdated language for Mohammed, Islam and Mecca that is considered offensive, and plays it fast and loose with the actual historical bits.

I dunno. I'm reading more about it rn and it's like if the Pope issued a Crusade against Dan Brown for the questionable historical interpretations in the Da Vinci Code.

edit: okay I've read a few non-Wikipedia sources and I haven't seen anything besides what I already laid out. The author was raised Muslim, and it seems that he had some knowledge about Islam-adjacent conspiracy theories that he worked into the narrative, which word of mouth spun into wild accusations that eventually reached Ayatollah Khomeini's ear.