Again, a statement made my a character being a reflection of their own arrogance and/or self-confidence. Completely different from when a character absolutely knows that they can do something. If Kakashi says that his Kamui is able to warp things into another dimension, but he has never shown in a panel that he can do it, his statements of his current abilities still make what he says canon. And it is canon, because Manga allowed him an opportunity to showcase his ability. Tsunade, however, was never given this opportunity. But again, the thread, "can she regenerate" not, "has she ever regenerated".