They will have to travel through days of starvation, over the sea lane to Port Nebel, before they can attack. Have fun with that.
They'd been promising for weeks to attack Aurvandil, and then turn and attack D'Hara instead? Cowards.
It has nothing to do with being afraid of Aurvandil, though I suppose there is an element of cowardice involved. However, it's more like political cowardice, since attacking Aurvandil without ripping Luria apart might have required some tricky maneuvering internally.
No, the reasons given for attacking D'Hara are real, and only the internal politics weighed heavier, not worries that Aurvandil would chew us up and spit us out.
