They clearly were under the impression that they were sovereign states, no?Article II. Each state retains its sovereignty, freedom, and independence, and every Power, Jurisdiction, and right, which is not by this confederation expressly delegated to the United States, in Congress assembled.
The constitution lays out the rules for the federation and the rules for the member states. I see no prohibition on any of the member states from leaving the union, in which case, none of the lines in the constituion would apply to them, since they would not be members. The constitution only applies to those who belong to the union.Well what succession is is a declaration that you are rejecting the whole constitution. You are saying you will no longer follow anything it says in there. So really, every line of the constitution prohibits succession. For example, the constitution says that the states must appoint representatives and senators. They're breaking that by succeding. It says that the states can't deny anyone due process under the constitution, but if they're no longer recognizing the constitution, then they aren't doing that. Etc. There is nothing more illegal than rejecting the law entirely.
A treaty with no language specifically describing its termination is assumed to be "at will". This means that the members of the treaty may leave whenever they wish.They have agreed to be bound by the constitution, so they waived their power to do things the constitution prohibits.