The author of one study says that; it's not a consensus of intelligence specialists. From what I know, textbooks are usually carefully reviewed by experts; in fact, they likely are far better reviewed than this study, which wasn't even published in a peer-reviewed journal [EDIT: As pointed out below, it is in fact peer reviewed; sorry, my mistake. I still expect textbooks to be more carefully reviewed than one random paper.]
From the article:
Reporting their findings in an open-access article in Archives of Scientific Psychology ...