I don't actually think this was deliberate—the author of the new book is a scholar I respect a lot. I think it's a weird side effect of citation practices that, for largely good reasons, prioritize citing the original source over citing the secondary source that led you to it

(It may also have been due to the author of the new academic book not being a historian—I think those of us who aren't historians sometimes overlook how much work it takes to map out these explanatory "chains" and how much credit is owed to the people who do it)