The new book does cite this historian, but only as a commentator on ONE of the primary sources—as someone who noted one link in the chain, not as the person who assembled the chain itself. The implication is then that the author of the NEW book was the first to put the chain together

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