That is my understanding. So long as the Constitution has jurisdiction, it was legally impossible to abolish it. It would have required an amendment, or states voluntarily giving it up, though I don't know what they could have done about the "privileges and immunities clause."
If all the states outlawed slavery then para.3 Sec.2 Art. IV becomes extraneous verbiage in the Constitution.
Under that clause, a state is only required to return a run-away slave. it does not prevent a state from making slavery illegal within the borders of that state.