I think this is one of the things that I didn’t like that I couldn’t quite put my finger on. Thinking back on it now, I think the author was probably trying to take a neutral stance on things. I feel like it’s a pretty normal thing to want to try to keep a non-fiction book as unbiased as possible, but (1) writing anything in an unbiased way is nearly impossible, and (2) the way the author of this book did it, as you said, was inconsistent and a little inaccurate. I might be interpreting the author’s intentions wrong, but if I’m not, I’d rather her straight-up say “I believe ABC because XYZ. However, other people believe QRS because EFG.”
In my opinion, as an author, if you don’t make your biases clear and then try to present other views as neutrally as possible, it comes across as not-genuine, wishy-washy, sketchy, and inconsistent. Not admitting one’s biases can cause more issues than it solves, especially when it comes to one’s credibility as a non-fiction writer.
I could go on and on about this sort of thing, but for anyone who’s interested in writing, especially non-fiction, I’d VERY highly recommend the book They Say / I Say: The Moves That Matter in Academic Writing* by Gerald Graff and Cathy Birkenstein!