@NN674221 at the time, Hiruzen was supposed to be the strongest Hokage besides Minato; that’s why they say he surpassed his predecessors
That was clearly retconned later because Hashirama is Hashirama, but that’s the most likely reason it was written that way
Later on they say that it just failed since Minato was in RDS, but by that logic Orochimaru shouldn’t have been able to prepare it in the first place, so that’s just a plot hole