You should insist that your mortage lender removes false info from the credit report. Requesting anything from CB IME is less than useless -- they would likely just update the entry date, not its content. Remember, you're not a customer of a CB, but a product. If your mortage lender does not immediately commit to correcting the error, file a CFPB complaint.
In the meantime, you may still try that offer. If you've been a good customer of AmEx, your current score is not automatically a deal breaker.