That's why I also said it's designed to elicit fear in the audience, which is the primary distinction of a horror movie.
https://www.irishtimes.com/blogs/scr...a-horror-film/
The reason I say it's not subjective is because it's about what the filmmakers intended. Beck & Woods wrote a script intended to be a horror movie, John Krasinski adapted and filmed it as a horror movie, and the studio marketed it as a horror movie... so it's a horror movie.
Like, you can argue about whether Fight Club is a comedy—it's a funny movie but doesn't use jokes as its primary driver—but you can't argue whether or not Knocked Up is a comedy. It may not have made you laugh, but it is, by design, a comedy.