Per [1], it was a special toilet designed to support flushing at depth (I guess it makes sense that it's a tough problem to solve when the sub is under pressure). I guess previous iterations of U-boats had to surface to flush. It does seem like keeping it in a tank until the next surface would be a better solution though. I mean, they have to surface for other reasons too, right?

[1] https://warisboring.com/the-high-tech-toilet-that-destroyed-...