Originally Posted by
jdrtravel
It's actually totally unclear in a connecting situation where DL is liable for the delay. It reads "Customers who fail to complete the check-in time limits" but this is not an issue of check in. OP was checked in and already "in flight."
But the bolded part is independent of (“in addition to”) the check in time limits and there is no exception stated for connecting tickets vs originating flights. It is a stand alone requirement. You are required to be at the gate by T-15.
“
Additionally, you are required to be at the gate and ready to board at least 15 minutes before your scheduled departure time. “