For all the people who rant about Amex and other "high end" card issuers, this is where it pays off. Amex had no legal obligation to permit a "redispute," but had it not done so, there never would have been the opportunity for AM to fail to respond and OP would have been SOL.

Also points out that many carriers are perfectly prepared to respond with a solid "no" in the face of a DOT complaint. Contrary to FT conventional wisdom.