I think it’s a little bizarre to suggest that Stephen King is unaware of F/SF/H short fiction markets. No, he doesn’t know about every last little one of them, but he has published in plenty of them, and I’ve read endorsements of some of them by him over the years. I would assume his non-mentioning of these markets is an… understated… comment on his part of their popularity among and importance to the general population of readers.
I agree that it’s a bit bizarre to suggest that; King certainly knows all about genre markets. He’s published in F&SF a number of times, and once (a few years ago) called it "the best fiction magazine in America." He also reprinted a story from F&SF in BASS (and listed four others on the 100 distinguished list), so obviously he’s aware of genre magazines.
Flipping through the back of BASS, where it lists the addresses for magazines publishing short stories (which I assume to be their "magazines received and considered" section), I did notice that while F&SF was listed, Asimov’s, Analog, and Realms of Fantasy were not. I didn’t comb through it looking for other genre magazines, but if F&SF is the only genre magazine BASS considered, that’s curious.