I'm not sure they're saying it's wrong more that the change was imposed externally by the victorious union rather than actually being arrived at so the question was never really settled. Looking at the history it looks clear it was. After reconstruction was halted and southern states weren't forced to allow black politicians and voting you get the decades of segregation, Jim Crowe laws, etc that followed until the civil rights act forced equal treatment under the law. Civil rights were never willingly given by the southern states.
I'm not sure they're saying it's wrong more that the change was imposed externally by the victorious union rather than actually being arrived at so the question was never really settled. Looking at the history it looks clear it was. After reconstruction was halted and southern states weren't forced to allow black politicians and voting you get the decades of segregation, Jim Crowe laws, etc that followed until the civil rights act forced equal treatment under the law. Civil rights were never willingly given by the southern states.