I think it's considered a reverse harem because FL don't choose a ML. And I didn't read the novel so I can't be sure but I guess we will not see any romantic scenes with any potential ML's, just friendly vibes/friendzone.
Because story build around Hailey taking revenge (potential ML's with us mostly as supporting characters)
There are harem stories where the mc don't choose any of the love interests in the end. Harem plots don't always end with mc choosing everyone in a poly relationship, but rather cuz mc was simply not interested enough to choose one of them, at least from some stories I've read. There's ones where the mc's choice to reject/choose romance never really affect the plot heavily. It's more like subplot to be honest where there isnt any serious romantic developments.
I haven't read the novel of this so it might change later on, but as of now I don't see a lot of serious romantic tension going on between Hailey and any of the male leads. Right now, the story is just mainly focused on the revenge, rather than the romantic aspect and without having it affect the plot or mc's decisions too much. Honestly sometimes, i even forget this story has a romance genre. Again, i haven't read the novel either so might be wrong. But based on the way the story is progressing around the revenge, i doubt the romance parts would have too much influence on the current plot.

It has a very good plot until I heard it is a harem one. You could build the story many ways but ending it with harem will not be satisfying for me. She could choose one or not anyone but choosing everyone is i think clashing with her morals. Like I know she is weak to handsome faces but polygamy is totally othe thing.. dropping