Cuz being split in half and pinned by a tree isn't a situation that would call for a last resort technique.
Anyway it would have made more sense for her to have a sage mode but she already revealed Byakugou in this arc as her big trump card. To reveal another one would just be...well...yeah you get the point.
byakugou was her second strongest jutsu. OK for starters why she has it 100% the 3 ninjas are called LEGENDARY SANNINS.
the places are called LEGENDARY SAGES, which are equal to each other, same thing applies for the sannins.
the fact that SHIKKOTSURIN was mentioned with the other 2 SECRET PLACES, (which belongs to her other 2 friends ) IS THE FACT THAT SHE HAS A SM.
lets look at requirements for SM now.
high chackra levels-------------------Tsunade is a tank
concentration for absorbing nature energy---------she has precise chackra control, which deals with a lot of concentration to perform, multiple tasks like her slug katsuyu.
her attribute towards having it
YIN SEAL
MYSTICAL PALM-------sounds the same when a sage user absorbs nature energy. so she does have it and she will use it TO HEAL the kages, then FIGHT edo madara once more. we are about to see the true power of FEMALE SANNIN, and the rest of her slugs/snails that she kept hidden all this time
