Judging from the timing of him coming out to claim that (when Trump is running for presidency), it is very likely that he might have his own hidden agenda or could be doing it for the democrats. It would be more believable if he had come out earlier.
Also, most of the famous people didn't actually write their books. When they say they 'write' their books, it doesn't mean literal writing. It is true for Hillary Clinton, Michele Obama and the likes. Rather that arguing about these trivial things, why not discuss about the books' content and which book brings more value.