The shortest pamphlet ever printed on the ratification debates would show that that statement is just flatheaded and wrong. As if you'd care, when you're slamming the South again.
There were concentrations of slaves as high in southern NY state as in any southern state then and States' Righters there included George Clinton, the beneficiary of their Masters' votes. At that time there was legalized slavery in almost all states.
This refers to the power structure supporting slavery north and south which was the core of the democrat party of that day, the anti-federalists not all of whom were from the south.