According to Chief Justice Taney (a Democrat), slaves were not people either.
Actually, the US Constitution made it a fact that slaves were not citizens, even after being freed, CJ Taney was the mouthpiece for the SCOTUS since at that time only one opinion was made-the winning one, and the finding was written by the CJ, like it or not. This prompted the 14th Amendment, which was written to correct that and grant former slaves US Citizenship and thier children the same. No reference to the humanhood of slaves/former slaves/children of the same was made in either document....
We know that the US Constitution as ratified was a compromise to strengthen the confederation, and that all southern/slave states were at risk of balking and walking away from any form of union (federation or confederation) unless the slave issue was punted...
That was a most unfortunate compromise and unfortunately ambiguous wording. Slaveholding States were trying to get their slaves counted to increase their representation in Congress, which non-slave States opposed. Slaves were not legally citizens, which would not happen until after emancipation; so they could not be termed "citizens" for purposes of representation. Hence the misleading legal terms "person" or persons under the law" -- not "people", which denoted more than one human being, or "a people", which meant an ethnic group or nation.
A "person under the law" is an abstraction, It can also be a corporation.
That said, I am in no way endorsing slavery, then or any other time. Just a discussion of the different point of view then, when the Court still tried to stick to dry legal interpretations, not social engineering. Their wording was clumsy and naïve in light of the explosive consequences.