Well, they were not to be under the jurisdiction of any state. If DC were a state itself, it would be under the jurisdiction of the new state.
(God knows what they would call it)
So it would appear, at first blush, to be unconstitutional.
But, then again, as it turns out, the Obamacare Mandate is a tax, so what do I know...
But the new state couldn’t elect a state legislature that would have any authority within the confines of that 100 foot square area right? The Constitution says that the congress would have “exclusive Legislation in all cases Whatsoever”. So the new state would be powerless to pass or enforce any laws that would fall within DC anyway.