Originally Posted by
HDQDD
Technically if they’re HKed on a flight (HK = holding conformed) and they aren’t allowed to board its a DB. In OP’s case its unclear if they were HKed or some other non-confirmed/ticketed status.
During IROPs it is customary for airlines to call each other before potentially overbooking another airline’s flight, but it is NOT required. If an airline accepts a booking request with an HK response, they are now that airline’s responsibility.
As I said in the thread on this incident in the AS forum, it's possible that AS didn't reissue the ticket correctly for the DL flight and attach it properly to the DL reservation, so that sending the ticket to DL could have failed even if there had been a reservation on DL. AFAIK this would be consistent with the boarding pass being issued, but the OP later being denied boarding.
Without a correct reservation and matching ticket, it's certainly not an IDB.