I've been thinking, this flashback began with Kin'emon reminiscing about Oden. That reminiscence makes sense for now because Momonusuke wasn't born yet.
But the Kaido invasion part should be flashbacked by Momonusuke/Hiyori because his/her point of view should be the most heartfelt.
I think this flashback may end with Oden telling his retainers about why the borders were closed. The Kaido invasion flashback may be done from Momonusuke/Hiyori point.