Not saying Vonnegut couldn't have done better historical research, but Slaughterhouse 5 was not intended as historical analysis and is much more a psychological novel about PTSD and the effects of war on the mind of someone who lived through the brutality taken from his personal experience of being a POW during the Dresden bombing. He picks up what amounts to an early pop-history American source and doesn't really critically analyze it - just takes at face value its account of the event that he mostly focuses on from his personal, micro-perspective. I don't know if later in life he was confronted with more accurate accounts of the Dresden bombing and whether he commented on the inaccuracy of his books, but you can understand the literary appeal to a surviver of the Dresden bombing being presented with an official history that confirms what he emotionally felt while in the middle of it. He even presents it that way in the first chapter - describing himself and his army buddy as basically ignorant to the macro history of the event until they crack open a book decades later that describes it that way. When the "author" of the referenced book appears in the story itself, he is presented as one of the most deplorable characters confronted in the book. Essentially a bloodthirsty maniac that is both unapologetic while being aware that his conclusions are unsupportable (feeling the need to get confirmation of his statements of belief from a person that he does not even acknowledge to be conscious or cognizant). All that is to say, if the only thing one takes from Slaughterhouse 5 was that it is "bad history" and somehow nazi-aplogia for exaggerating the extent of death in Dresden, or worse, if someone avoids the book altogether because of the accusation, they are really missing out.