The quote is from the Virginia Constitutional Ratification Convention. There was no Bill of Rights yet. The federalists were opposing having one.
He was asking for one.
One of his expressed reasons- as you can see in the quoted remarks- was to limit the meaning of the Supremacy Clause so the state Bill of Rights would be protected from the federal government.
Agreed. But more specifically, his remarks were directed toward the power of the Federal Congress, not the Supreme Court. As you pointed out in a prior post, his concern was that federal courts might not be diligent enough in restraining Congress.
The point of his speech was to ensure that the Rights of Virginians were secure. He knew that the Supreme Court was to be given original jurisdiction in all Cases in which a State was a party.
If he were worried about the USSC, I would expect that he would have made mention of Article III.
Did Mason ever warn against USSC original jurisdiction in all Cases involving States?
Also, would a USSC interpretation that says the Second Amendment applies to the States make the RKBA of Virginians less secure?