Horror and humor actually go hand-in-hand together. It is a fact of human psychology that people faced by horrible circumstances will often make a joke in an attempt to alleviate their own stress at the time. Throwing in a stupid pun, a funny comment, an attempt at humor, or having a character do something hilarious is the horror writer's way of breaking tension in a moment of suspense, thus extending the reader's roller coaster ride in the story.
Horror and humor are old, old friends artistically. I think it may be true that the FBer was trying to joke around and it may be that the OP here is just taking things way too seriously. In all honestly, lighten up. I can cite you over 150 professional authors who use humor in their science fiction, fantasy, and horror writing, and in their FaceBook posts. Just as an example, Glen Cook, who has written the serious-as-a-heart attack Black Company series sprinkles the occasional piece of humor throughout those novels. In addition, his Garrett, PI series is hilarious at points while being both fantasy and noir detective novels. David Gerrold sprinkles humor in almost every chapter of his The War Against The Chtorr science fiction novels. Larry Niven's Ringworld is full of humorous comments made by characters at the most inappropriate times. Etc., etc., etc.
Horror and humor are not exclusive to horror novelists. It can be found virtually everywhere in writing if you take the time to look and don't take everything so seriously. Writing and reading should be fun activities as well as work (well, writing anyway).