I'd definitely say that Immortal Hulk is a horror comic. It's been quite disturbing at times, and is definitely not a superhero book. It's less of a superhero book that Moore's Swamp Thing, because at least Swamp Thing had morality. There's nothing heroic about this Hulk. There are more similarities, like between General Fortean and General Sunderland though, but that's also part and parcel of Hulk continuity, considering Thunderbolt Ross.
I'd also definitely agree about the Moore Swamp Thing comparisons. It's easily accomplishing the same things for the character, although the Hulk is a much more high-profile character,. If I were asked to compare it to another comic book, I wouldn't hesitate to say Moore/Bissette Swamp Thing.
Yes, I like this new trend at Marvel to look back at the origins of some of their classic characters and reinterpret them in a completely new direction. It's a good usage of continuity without just writing a tired rehash of the same stories that were done better a few decades ago.
Those early Stan Lee/Jack Kirby Hulk comics, they were definitely not a superhero comic. The Hulk has very firm roots in the horror genre, although it didn't last very long before Lee decided to try to make the Hulk a lot more sentimental.
Ewing is tapping in to those earliest hints about the direction that the Hulk comic could have gone under Stan Lee, but going somewhere very dark that Lee would have never touched. A sort of Stan Lee meets Alan Moore mysticism.
Hickman's X-Men run has been mining some of that same territory. I don't care what people say, I thought there were some good stories in those first couple of Lee/Kirby X-Men comics.
That very first issue of X-Men, there's something creepy and unsettling about the mutants when we first meet them. If you were reading that comic in 1963, it would have been a different experience. "Who is this bald guy, calling out to his X-Men? What is this all about?".
Hickman also tapped in to that feeling of weirdness that was there in the first Lee/Kirby X-Men comic, but has also mixed in elements of the character-defining Chris Claremont run and Morrison run, to tell a different story about mutants.
I like how these creators are going back to what Lee (and usually Kirby) gave us on the comic page, before Lee really figured out what he was doing in creating a superhero universe, and backed away from some of the ideas from the collective unconscious that he seemed to be tapping.
Anyway, back to the Immortal Hulk, yeah, this is not only the best "superhero" comic being published today, it's the best horror comic that I've seen in recent years.