I am guessing he means it is unfortunate because it was considered a good book for critical thinking but over time people have come to realize a lot of its stud

I am guessing he means it is unfortunate because it was considered a good book for critical thinking but over time people have come to realize a lot of its studies, reasoning, and arguments are flawed - but many people have already read the book without knowing that. I've never read the book, so I don't have an opinion on it.