The German planes cannot hit the Russian submarine. They would only be useful to help kill the cruiser.
If both cruisers are lost and the battle just comes down to the German sub attacking a transport, then the planes are useless and the transport is killed automatically and the battle is over.
He doesn’t have to roll a hit to get the transport.
The rules make this automatic since there is no other way that the battle could end, as the planes can’t hit the sub and the sub can’t hit the planes, and the transport obviously is defenseless.
So even if the transport wasn’t just killed automatically, the owner of the German sub would just have to keep rolling until he got a 2 or 1 and the transport gets killed anyway.