Why a cork-filled battleship target did not need to sink

Was the old battleship packed with cork successfully made into an unsinkable target?

Cork made the target very difficult to sink. The exercise measured confirmed hits, so firing did not have to continue until the ship was destroyed.

Cork filling made the old battleship difficult to sink, but it did not make her literally unsinkable. The added resilience let the target survive repeated firing practice. Firing stopped once observers could establish that the target had been hit rather than after the ship was destroyed. The experiment therefore measured gunnery accuracy instead of terminal damage. Its success can only be judged against that test objective.