While states have historically entered into confederations, alliances, compacts, treaties, and other agreements after seceding, this is not the secession itself nor is it a requisite part of secession. Therefore, the provision does not prohibit secession but only prohibits such actions while the state is a member of the Union. Since none of the aforementioned actions which are prohibited is a requisite of secession, they don’t make secession any more unconstitutional than, say, the prohibition of the states to grant titles of nobility.
