It's not one expert against many, it's people outside a speciality refusing to accept the consensus from within that speciality.
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 ...
> Archives is staunchly peer reviewed, and the standards for publication in Archives are as high as those for any other APA journal. Therefore, only those manuscripts that are well-written and present research of the highest quality will be accepted for publication. Rejection rates are similar to those of other APA journals.
"Open access" is not the same as "not peer reviewed." There is no subscription required to get the articles in the journal, but it is peer-reviewed.
From the page for the Warne et al. paper from the OP link (http://psycnet.apa.org/record/2018-07714-001):
>Publication Type: Journal; Peer Reviewed Journal
From the publication announcing "Archives of Scientific Psychology" (http://psycnet.apa.org/fulltext/2013-03009-001.pdf):
>Archives of Scientific Psychology uses peer review to determine whether a submitted manuscript is suitable for publication. Authors are free to submit manuscripts that reveal their identity or that are prepared to keep reviewers unaware of who they are. We adopted this policy because we are unconvinced that evidence on peer review rules out the possibility that some reviewers’ evaluations of manuscripts are influenced by characteristics of authors (Shatz, 2004; Weller, 2001). Likewise, reviewers will be given the option of revealing themselves to the authors.
I've seen far too many minor-to-grave errors in almost every textbook I've read to believe that -- and I wasn't any kind of expert in most of the fields concerned.