War can wait.
Flashback is more important to actually deliver impact on the war itself.
Without Oden flashback, without seeing the scabbard personality around Oden. Without seeing the smallest details about the group of allied Samurai the SHs are allied with.
NOTHING from the war around them will actually hold any impact.
People will just want to fast forward their screen time fight during the war to get to (Luffy/Law/Zoro/Sanji).
So, we have to get close to the 9 red scabbard so that, when Kinemon and co get a proper impact during the war, and so that the fans don't act like a whiny complaining viewers who say "I don't care about their fights, just skip them, and give me Luffy vs Kaido or Zoro vs XXX"
That shit will turn the war which is "supposedly better than Marineford" into just fans wanting two battles and skip others.
However, Oda has to make us attached to most prominant characters in the flashback.
It's the right way to actually make all the fights included during the war has a "meaning" and get a great deal of impact.
Just think of the SH grand fleet for a second during Dressrosa as an example.
Oda showed their battles and fights WITHOUT us having their backstory or their importance for the story. And the fans during Dressrosa couldn't handle a chapter or two for those characters having a battle of their own. People were like "I don't care about them, just skip their fight and show me the SH crew fights"
And after the arc finished and it turned out that Oda made all that screentime for them for a reason, the formation of SH grand fleet. Those same people retracted on what they said and started to understand the reason behind the amount of screentime.
So, NOO. Flashback first before the war begins. We need to get attached to the 9 red Scabbard, and we see who might actually be their matching enemies from the enemy side "Just like how Inu and Neku vs Jack was built in a right way" We need for the others to have specific characters from enemy side being built as an obstacle. And along with the tragedy they will have during the flashback and losing Oden...etc. That will make us WANT AND DESIRE their battles and not be skipped.
So, I disagree wholeheartedly. IF Oda skipped the flashback before the war, Oda basically will ruin the war and how impactful each fight and clash we are about to see, the war will be RUINED