I tried to remember as I don't know the exact XKCD I read, as I was going through them from the first.
The basic complaint is some kind of "How are people like Hitler elected" statement was in the cartoon, and I knew the answer as I'd read up to 1939 of the book the Rise and Fall of the 3rd Reich, which is I hope not revisionist.
I think the cartoon said something about the history of the Nazi's, which I'd read about, and it was wrong or wide of the mark.
I'm a computer scientist with a humanities diploma (media studies) and I recently started reading more history books.