I hope it wasn't Clarkesworld. The first thing in their submission guidelines is not to send zombie stories.
Work that straddles genre and literary sells. There are short fiction outlets for that. There are novels like that. They don't sell as well as the straight genre stuff does when it comes to SF/F audiences. I think a "zombie" story without a zombie or zombie attack could be very interesting. If it's well done, I think it'll sell. The themes that arise around zombie-type stories could be dealt with without ever seeing a zombie in the work.
I think it could be an interesting idea if, say, everyone thinks there are zombies and acts accordingly, but in reality the zombies have ceased to exist or never existed in the first place. M. Night Shyamalan used a similar premise in The Village (though the feared creatures were your garden variety monsters, not zombies). This thread makes me want to pick up my Zombie Cupcake Romance short story again. I missed the deadline for the anthology I had started writing it for, but I always thought it might make a cute Halloween-themed giveaway.
I figured--I was just joking, because I think it is funny Clarkesworld put that in their guidelines as the first thing not to do (although I guess they're not absolute about it). I wouldn't assume the market you targeted would never buy a zombie story. Instead, I'd assume they get TONS of them and take a dim view of each new one they receive, which is also what likely prompted Clarkesworld to mention it as their first no-no.
They can be either Sci-fi or Fantasy, but they have become a huge cliche these days. Zombies are like an extremely overworn pair of undies, and have little left to do with them before they have to be thrown out due to stains and holes.
Not true, it's simply a matter of the time. Mary Shelly had as much grasp on the power of electricity than most scifi writers have a grasp on quantum mechanics. Remember, Frankestein was written fifty years before Maxwell's equations were derived and 40 years before On the Origin of Species. At the time, scientists were still debating spontaneous generation. The science is all there, we just now know that it's all wrong.
I'm okay with some of the stuff I write not being popular. Most of the stuff I write is not popular and doesn't sell. But certain types of stories are easier for me to write and take risks with. I know the zombie thing has been done to death, but each time I write a story about zombies I try to do it in a way that I haven't ever seen and in a way I had never thought of. And I don't really worry about messing it up. After all, it's just a zombie story.
I don't subscribe to idea that a genre or premise can be "done to death." If I have a zombie story that I'm passionate about then by God I'm going to tell it. Batman may have thousands of adventures under his utility belt, but imo he has an infinite more waiting to be told. Same for zombies. Same for dragons. You get the idea.
A zombie itself isn't science fiction, but the method of its creation might be. A zombie is usually a horror or supernatural trope. After all, it's a walking corpse.