her Yin Seal is still intact, that would mean that she wasn't able to use it when Madara bisected her.. what i can think off is, Madara attacked her first, seeing how the other kages are already severely weakened and fatigued.. and she just recovered her chakra after using Byakugou for an extended amount of time.. Tsunade can't heal herself because a giant tree is separating her upper and lower half, so therefore, she can't resume healing as long as the tree is there.. she and Katsuyu had already confirmed that healing her bisected body is not even a problem, but she just chose to attend to the other kages first since they don't have the same longevity as her.. she is alive not because of her med skills, but due to her longevity and life force being a senju/uzumaki hybrid..
plus, she still alive isn't she? so she still managed to live up to her hype of not being able to be killed in battle, as even if she dies, it is due to her attending to others first before herself..
