In the course of the time it took for Aku to say, "Okay, time to kill Jack" 5 different militia's were able to make it to Aku's fortress from all across the setting in order to try and stop it. The others left Aku behind in the future, so either Samurai Jack is a multi-string timeline series, and Aku's going to ass-fuck everyone for eternity since the guy with the sword is gone, or it's a single timeline series with Jack destroying eveything he's tried to protect. Ashi also de-existed just in time to fuck up a wedding that probably took days, or even months worth of time to prepare. Not a moment sooner, and not a moment later. Maybe she pulled a Tenchi Muyo and willed herself into existence until the end, but how the heck would the audience know that.
The series of actions that took place were logical enough, but the short amount of time in which everything took place really didn't help the ending. If the ending were a two-parter, and we could actually have the army prepare themselves and arrive in one part, then I wouldn't be here questioning how they got here so quickly. Have the fight and wedding take place in part two, with Jack killing BOTH Aku's and we'd be golden.