The director himself has said Zombi Child is a "horror" movie, but I think this is really true only in the most literal sense. Bonello filmed much of the movie on location in Haiti and the rest, presumably, in France. This fact of production - filming scenes that take place in 1962 Haiti, and some that take place in a contemporary Haiti which doesn't seem to have changed much; and filming most of the movies contemporary moments in modern France - encapsulates the movie's thematic intent. The latter are firmly rooted in colonialism, the ongoing legacy of slavery, and the cultural dialectic between assimilationism and the fierce preservation of traditionalism (arguably, two different kinds of survival, which is also a core theme: surviving the multi-generational damage of colonial )oppression.
If this all sounds deep, it is. This is a brief film, around 85 minutes, and the first 60 contain very little of what could be considered horror in the sense of "horror movie." The horror in the first two thirds of the movie is purely thematic and a historical. It isn't until the last part of the movie where Zombi Child's oversimplified classification of a horror movie becomes evident. In other words, those going into this movie looking for a traditional zombie flick are likely to be disappointed. It's really more of a drama in which certain horror elements come into play, but almost purely as metaphor.