Why would they need to claim that? The words speak for themselves.
Those who would believe otherwise must prove the founders did not intend for people to have independence if they wanted it.
Rather, Founders like Madison explained their Declaration idea as: when a legal contract is broken that relieves the parties from their legal obligations to honor it.
This principle is correct, but is not relevant to a right to independence which people have anyway.
But supposing this is a requirement in your mind, then I will point out that states' refusal to enforce Article 4, Section 2 constitutes such a "breach of contract."
Therefore even under your legal theory, the Confederates had a right to leave.
And "broken" refers to such major events as spelled out in their Declaration of Independence.
Fine. I'll go you one further. Here is one of the major events as spelled out in the Declaration of Independence.
"He has excited domestic Insurrections amongst us...
And here is what a conspiracy of prominent gentlemen in Massachusetts et al did.
So with Northern states having engaged in the exact same activity as listed as a "cause" in the Declaration of Independence, under BroJoeK's theory, they still had a right to leave.
Glad we could agree that they had a right to leave BroJoeK.