All ships SINK until the point they have displaced a weight of water equal to their own weight. And the term used is actually DISPLACEMENT.
TO lighten this sub by making it 70 tons lighter, may greatly reduce its effectiveness. It could be simpler, and quite a bit less costly to INCREASE the size of the hull so its displacement floated the sub at a “normal” waterline. Filling the tanks causes submergence, and pumping tanks lightens the boat and it surfaces. Just thinking.
Per the article, they are thinking about making it longer.