It's best to read the preface to that book, too, because I don't want to redefine what he is saying. People didn't want to admit its existence. Well, I had to mention horror. I think it's more apt to describe it as supernatural thrillers. I craft a lot of mystery in them, also a lot of adventure.
Mystery is the staple that has consistently sold, until recently when romance overtook it.
Mystery has historically been the mainstream of publishers. Now romance is, of course, pushing it aside. But mystery requires cerebral exercise. At least good mystery does. I don't know about romance.