He learned the technique the night minato used it to seal the nine tails and if minato doesnt use it to seal the nine tail then by definition Sarutobi hasnt learned it
Tobi's Bijuus nuke Sarutobi, Tobirama, Tsunade, and Minato with 6 mountain busting Bijuu Damas. Madara clears out any survivors with Perfect Susanoo and also holds off Hashirama long enough for Tobi to assist him.