None of this has to do with the simple fact that DL does not contract or otherwise agree to maintain a specific schedule and that the customer expressly agrees that DL may not be able to do so.
If you have a ticket contract which includes provisions by which DL agrees to run within 5 minutes of schedule, you have a beef. Otherwise, the assertion that DL has contractually committed to a specific schedule means that you either disregarded the contract when you purchased or read it but choose to ignore it now.