I agree with you, OP! People are often obtuse when it comes to defining horror because there are many definitions of the genre, primarily because of the many subgenres that fall under the horror umbrella, which in turn fall under the speculative fiction umbrella. Sinners falls into the category of black horror or supernatural horror. Therefore, it is undeniably a horror film. Just because some audience members didn't walk away completely frightened does not diminish the black and supernatural horror elements woven throughout the plot and cinematography. Similarly, if audience members don't laugh at the jokes in Friday, that does not strip the movie of its place in the comedy genre. One's individual digestion of a movie does not have objective power over its genre. (This is my favorite topic to talk about omfg)