The catch with most of those people, I've found, is that there is a big difference between having a PhD in the education version of a discipline and in having a regular PhD in the discipline.
I've not encountered the latter in secondary textbook authorships (nevermind promotion) in a very long time.
I'll bet the guy's PhD was in "Math Education" and that he's taught TEACHERS at the college level-- maybe even within a math department, but still, that is not the same thing as knowing the discipline like colleagues that teach math to STEM majors and math graduate students.
Where that kind of thing really shows up, in our own experience, is in assessment. Non-subject experts are just fine with rewriting questions to suit various levels of Bloom's, but without really understanding why they've just made the question ludicrous or worse from a subject standpoint.
