You've offered some good points, Peter. In general, I agree with you. Requiring a 2/3 majority to actually leave doesn't seem unreasonable. But I'm fine with a majority of a congregation deciding to alter their relationship with the ELCA, including withholding funds and removing the logo from signs, stationary, and websites.
However, in the case of St. Peter's, I think the congregation took the correct action. They did pass both votes with 2/3 majorities, but the synod decided to block it. So technically, they are still on the ELCA rolls, and the Bishops can pretend to speak on their behalf when they make social statements.