That’s parsing semantics. Dirty pool dude.

3/5 representation equates to 3/5 personage. It’s historical and widely accepted fact. De facto, your representation is a measure of how human you are. Not having equal representation makes you less human than someone with full representation.

No outlawing or barring slavery, regardless of custom, has the force of condoning it. Search and seizure used to be custom too, but the Constitution forbade it. Why would the founders have done such a thing unless they disagreed with the policy despite the custom? Same thing for locking up political opponents for speech unflattering to the crown, persecution due to religious beliefs, denying weapons to the populace, etc. Pretty much all of the Bill of Rights were custom that the founders disagreed with and so barred from becoming law.