Charlie Kaufman is probably best described as a surrealist filmmaker and he defies genre definitions. I'm Thinking of Ending Things is based on a horror novel, which is a little more conventional than the film, though the conceit of the film is the same. If it's horror, then it's existential horror rather than horror of the jump scare or suspense variety.
I had an initially negative reaction to I'm Thinking if Ending Things, it is hard work at times but the film stayed with me. I'll probably give it another watch soon, now that I know what's going on with the characters. I felt similarly about his debut as a director, Synecdoche New York, which I now love. Before he also directed, Kaufman was the only US screenwriter who was considered more the auteur of the films he was involved with, than the directors who adapted his screenplay, his voice is that distinctive. Being John Malkovich and Eternal Sunshine of the Spotless Mind are based on his screenplays and they are more accessible than his more recent films.
Netflix way of promoting films is frequently misleading and can be controversial, like its tasteless marketing of the otherwise well received film Cuties which lead to a massive backlash. I would never just go by how Netflix describes a film, I'd have a look online whether something might be up my street if I don't know much about it. Kaufman has been around for over two decades and a quick search should make it clear, that what you are in for is something less conventional and more challenging than a straightforward genre film.
