They could rule that it's invalid because it was not legally adopted, but once an amendment is accepted as valid, it's part of the Constitution. The Constitution cannot be unconstitutional, by definition.
The 18th Amendment was a dumb idea, but it was legal. It was proposed by Congress and adopted by the states in the prescribed manner. SCOTUS or any other court had no authority to override it -- the black-letter law was before them, and they were bound by it.
See my post #230.