Castro took power by force, an unelected dictator.
The southern states had democratically elected legislatures, and the states exercised their right to abstain from the union.
If the US Government looks at these two as the same, shame on them.
“The federal government, then, appears to be the organ through which the united republics communicate with foreign nations and with each other. Their submission to its operation is voluntary: its councils, its engagements, its authority are theirs, modified, and united. Its sovereignty is an emanation from theirs, not a flame by which they have been consumed, nor a vortex in which they are swallowed up. Each is still a perfect state, still sovereign, still independent, and still capable, should the situation require, to resume the exercise of its functions as such in the most unlimited extent.” George Tucker, 1803.
But Tucker was not a Founder -- he neither helped write nor voted for the US Constitution.
Tucker's fellow Virginian, James Madison, was a legitimate Founder, who both helped write and worked hard to ratify the new US Constitution in 1787-8.
So Madison's opinion on the subject of "disunion" matters, while Tucker's does not.
Madison, like all Founders believed that "disunion" was legitimate in two cases: 1) mutual consent or 2) federal abuse or usurpations of power.
They did not agree that secession "at pleasure" was acceptable.