States cannot, without consent of Congress, lay duties on imports from states.
If they cannot do them unilaterally while a part of the U.S. then implied in that is that they lack the power to do so merely by leaving as well.
You lost me. A State leaves the nation. Now as an independent country it cannot lay duties on imports from other countries?
Sure it can. But it cannot do so unilaterally while part of the U.S. It is an example of the fact that the Constitution forbids the states from taking unilateral actions which may have a negative impact on the interests of the other states. By implication those forbidden actions include leaving unilaterally as well.