Earlier quoted context omitted.
Amendments are done by the consent of 2/3s of the states, as is spelled out in the document that governs the union of such states. If you don't want to call that a "referenda", I think you're incorrect, but it doesn't really matter.
States != people. People have to vote directly in order for it to be considered a referendum.
It doesn't have to be people voting directly. It shouldn't be.