Answer:
Yes, it is valid.
Step-by-step explanation:
Article VI of the U.S. Constitution establishes that the Constitution, federal laws and treaties made should be considered "the supreme law of the land" (the highest law), and therefore, all states should be bound by them and no state law can take precedence over the most supreme authority.
Therefore, a valid treaty, that is to say, a constitutional treaty, is superior to any law that states may create, and since the state law substantially impedes imports from the foreign country that was agreed in the treaty, the President's proclamation nullifying the state law pursuant to the treaty is completely valid.