Slaves were better off as slaves than as free blacks in America and in Africa. In America, free former slaves were still being treated like slaves, just without a master. They barely earned enough to survive and had little to no rights. Slaves with a master could have a roof over their head. In Africa, where there was no modern technology to assist with the work. Individuals were subject to the whims of neighboring tribes and terrible living conditions.
When the authors said that, they meant that slavery