They managed it by compensating for the DRK. It's that simple.
DRK can do any content but the healers will have to work harder and the DPS will have to be better to get such a time.
You can not make the claim DRK is not worse than WAR and PLD. It has less defensive CD's less overall DPS and brings no raid utility.
They can still clear the content. The group is just simply worse off than with a PLD or WAR.