Yeah, in that case, it's hard to forgive. Some of it may be simple ignorance, of course; until very recently, Christianity did its best to edit Judaism out of its history, and the effects linger. It's easy to do a cursory reading of medieval history and just kind of entirely miss the fact that there was anyone in pre-Reformation Europe except Catholics, at all! But it sounds like the author put a fair amount of research into the world in other ways, so I'm not sure how much of an excuse that is in this case.

Though I've a lot more Ashkenazim than Aztec in my veins myself, the example I cited bothered me even more, probably because there is no possible way that anyone can fail to know, after more than an hour's reading on the subject, what actually happened when Cortes was identified with Quetzalcoatl.

The throwing-bits-together approach can be lots of fun, of course, but the reader has to be sure the author is doing it on purpose instead of just out of laziness.