If you mean applying constitutional law as written, then yes, they were.
The fugitive slave act requires exactly the same thing as Article IV, section 2 of the US Constitution.
They only created the fugitive slave act because states were deliberately ignoring the requirements of Article IV, section 2 of the US Constitution.
States are not required to enforce Federal Law.
One would have thought that the Southern States would have applauded this as a principal of “States Rights”