Domestic violence is pervasive even today. There is unfortunately nothing special about Fusiwe breaking the arm of his spouse in anger, it could have been some Dr. Frank in Minneapolis in the very same year instead.
The only difference is that tribes living in an open forest don't have much "familial secrets": nothing can happen behind closed doors if you live in a place with no doors to begin with. So everybody knew, and in absence of strong taboos to the contrary, such behavior was normalized.
It absolutely did. A violently broken arm with a stick can result in an open wound where the sharp end of the broken bone sticks out (an open fracture). This, happening in a rainforest with no doctors around, is a recipe for a deadly infection. We don't think about limb fractures per se as being too serious, but this is only true in the context of developed countries of the last 100 years or so.
Fusiwe absolutely could have caused a mortal injury to her, even though it wouldn't have been immediately fatal.
The only difference is that tribes living in an open forest don't have much "familial secrets": nothing can happen behind closed doors if you live in a place with no doors to begin with. So everybody knew, and in absence of strong taboos to the contrary, such behavior was normalized.