Fair enough: if there is evidence for that scale itself, then the author's credibility is far less relevant.

Thank you for prompting me to look for the actual instances of evidence-lacing for the author. Turns out, I'm wrong. I was too quick to challenge his credibility.

It is actually Martin Seligman, one of his contemporaries that allegedly churned out an empirically un-validated theories:

Please see this Wiki page. The last line in that paragraph is a disappointing 'These theories have not been empirically validated.'