The 10th Amendment is in plain english and does not mention the Union at all. Because it does not do this it also gives no power to the Federal Government to force a state to stay in the Union.
That's the entire premise of the 10th Amendment. If the power is not enumerated in it, it does not exist and belongs to the states and to the people.
Why would an amendment mention the Union? It merely referred to internal affairs of a state. Of course it does not give the federal government power to do anything that was not the point of writing it.
But it in no way affect the power of sovereignty which the fedgov possesses.