That's a matter of interpretation. Again, the Confederate constitution was very explicit about it, but the US Constitution can be interpreted to mean the same thing.
George Washington kept slaves in Pennsylvania after that state had passed a law making it illegal.
I think efforts to bar slavery in states run afoul of Article IV, which guarantees all the privileges and immunities of the citizens of the various states.
How can you have a system in which slaves are recognized as property by the entire Union, but then not allowed to be in some states, but allowed to be in others? This seems like a clear infringement of the immunities and privileges clause.
Other people felt states had a right to ban slavery within their borders and saw no conflict with the rights guaranteed to the citizens of other states.
Which view is correct is not completely clear. I think the US constitution can easily be interpreted to mean the same thing as this aspect of the Confederate constitution, but the Confederate constitution leaves no doubt at all.
That's what I mean when I say it is clearer on the point.
“George Washington kept slaves in Pennsylvania after that state had passed a law making it illegal”
In 1780,Pennsylvania passed a gradual emancipation law. If born to a slave woman in Pennsylvania, you became free at age 28. Washington moved his slaves approaching 28 out of the state.
If you were a slave born outside of Pennsylvania, you were free if in the state after 6 months. Washington would move the slaves to MD, or VA for a few days, then return them to Pennsylvania. In both cases he was complying with the Pennsylvania law.
By 1840, there were only 64 slaves in Pennsylvania.
You don’t think the founding fathers that were alive when states started passing laws to end slavery in their borders wouldn’t have said something about it being unconstitutional?