I disagree, and it's one of my favorite novels. That book is horrific because it's trying to convey the horrors of slavery and the thorny and fraught nature of escape. They've managed to free themselves of the physical shackles that bound them, but the psychic horrors remain with them, and likely will for generations to come. Beloved was born outside slavery, but the spectre of being sent back there killed her, and she literally embodies the lingering history of her family's trauma. It's uncomfortable and it should be uncomfortable, and to me that's why it's horror.