Link to mistaken fare policy statement from DOT:
https://www.transportation.gov/sites...t_05082015.pdf
Note footnote #4: The burden rests with the airline or seller of air transportation to prove to the Enforcement Office that an advertised fare and the resulting ticket sales constitutes a mistaken fare situation. If a sale does not qualify as a mistaken fare situation, the carrier or other seller of air transportation is bound by § 399.88.
If you file a complaint, I suggest specifically referencing footnote #4 of the policy statement, and asking the Enforcement Office to have Delta prove that these tickets sales were a mistaken fare situation. It may also help to reference data from post
338 and state that Delta typically has a wide price spread for ticket prices, and that it is not unreasonable to assume a $2700 ticket is a valid price.