It doesn't matter if the rule is the purple rock or not.

What matters it that Amanda believes Cirie's purple rock story as the reason Cirie voted for Amanda. Erik is left looking like a dick, because he was not smart enough to come up with a story to tell Amanda to make her feel better about him voting for her. Amanda think Cirie had not choice, but Erik is a dick for voting for her. Cirie outwited both Amanda and Erik.