So, based on what you wrote, Massachusetts could not make slavery illegal in Massachusetts. If you were in Alabama, slavery would still be legal...in Alabama.
I am not sure I understand what you wrote. Did the people of MA think they abolished slavery in Alabama? The only time that would come into play is if you brought your slave with you from Alabama to Boston. Although...I think it was assumed that slave remained the property of the owner.
Am I understanding this incorrectly?
It could bar the creation of new slaves in Massachusetts. It could do nothing about those that were already legal.
If you were in Alabama, slavery would still be legal...in Alabama.
So long as Alabama had laws that held them in bondage, they would still be bound by the laws of Alabama even if they were in Massachusetts.
The only time that would come into play is if you brought your slave with you from Alabama to Boston.
Pretty much. But could a state ban you from coming? I don't see how if the larger covenant that was the constitution was to be respected.
Am I understanding this incorrectly?
I think you are in the ball park. I don't see any wiggle room in Article IV section 2. You could stop people from creating slaves in your state, but you couldn't stop slaves that were held in bondage by the laws of other states.
But the powers that be in states like Massachusetts behaved as if they could free slaves, and they got away with it because there was no effort on the part of the Feds to stop them from doing it. Like I said, it was a lot like this "sanctuary city" nonsense repealing Federal Immigration laws while the Feds just sit around and let them get away with it. (Same with legal marijuana.)