You are correct for taking me to task for making this statement. Please allow me to "walk it back". The first part of the post I stand behind.
Restrictions on travel of an owner with his slave came into play later than Article IV. For example, in 1790 in Pennsylvania, if a slave spent 6 months in that State, they were automatically free. Even George and Martha were well aware of this:
Washington brought 8 slaves to Philadelphia in November 1790. Four of them, he returned to Mount Vernon in Spring 1791, before the 6-month deadline. Two of them were taken on a short trip to New Jersey by Martha Washington, which interrupted their 6-month residency. Two of them were trusted not to take advantage of the Pennsylvania law to seize their freedom.
Washington brought 8 slaves to Philadelphia in November 1790. Four of them, he returned to Mount Vernon in Spring 1791, before the 6-month deadline. Two of them were taken on a short trip to New Jersey by Martha Washington, which interrupted their 6-month residency. Two of them were trusted not to take advantage of the Pennsylvania law to seize their freedom.
I see two points regarding this. The first one is that should the issue have come to a lawsuit, a Federal Court might very well have ruled against the State. (Depends on whether they got a Liberal Judge or not.) I don't see how they can lawfully deprive a person of what was regarded by the law of that time as "property." It would be legally the same as arguing a man forfeits his wagon or his land if he crosses a state border. I don't see how that passes constitutional muster.
The Second point, is you are demonstrating how a man might continue slavery in a "free" state. What's to stop a man from maintaining a Plantation in Pennsylvania with a regular rotation of slaves in and out of the state?