Happy birthday, Knight-Commander! 
Only through rushed writing based on Varric's recollection of what happened, he can't be that awful or they wouldn't have hired him into the Inquisition, lol! He could tell that those 3 mages at the end of the game weren't blood mages. Hell, if I was him I'd still go down there and take a look, if only out of morbid curiosity! Better to know your "enemy" right?
No, he didn't argue with Meredith when she said there is no way to see if they were blood mages. He was just willing to believe they were telling the truth, because they were scared, and didn't want to die, and blood mages would probably summon demons to defend themselves, or used blood magic to mind-control the templars. So he was just going to trust them, and take that risk, because his heart told him so, I don't know 
But yeah, I hope Varric just forgot to mention something. Maybe Wilmod knocked Cullen out, or maybe Hawke didn't use magic, and was just beating abominations with her staff. Or maybe Cullen was just to focused on the abominations, got all those PTSD flashbacks, and completely didn't notice non-abominations, who knows?
It will be funny if he and Varric have banter about it in DAI. I hope they will mention it at least, or joke about it. Or we will hear that something completely different actually happened, and Varric just told Cassandra a story with a huge plot hole, and she will freak out about that. 