- #130
None of the others in soul society knew who he was when they saw him.
This is the first time most of them have seen the zero squad.
Isshin couldn't have been some random lower level soul reaper if he was able to get bankai and he probably fought with Bach when he used his Final Getsuga. That probably delayed Bach's plan for a few years too. Also Isshin knows quite a bit about soul society and soul reaper powers.
Now this part is just me guessing, based on other theories and hardly has any evidence.
Isshin somehow got related to a woman who Bach knows(possibly his daughter) and Isshin probably took her away to the human world after delaying Bach. When he lost his spiritual pressure, Bach couldn't chase him anymore. He probably met Urahara in the real world and told him about the soul king, or they probably met before when Urahara probably tried to infiltrate the dimension the Soul King lives in or maybe even before that.