Oh yeah, those are definitely flashback scenes. Like, it's clear that we're getting the story through flashbacks, through characters like the Deku Tree telling Link what happened (as he said he would in the trailer). How else would we get those scenes if the game starts with Link waking up with no recollection of anything that's happened?
This is basically what I'm thinking the basics of the story will end up being: The guardians are techno-magic tanks created to protect Hyrule, but at some point Ganon corrupts them and basically "reverses their programming." Link dies trying to protect Zelda during all this (it's a losing war, as Hyrule is already in ruins and the Master Sword has already rusted before Link wakes up), so the sages have no choice but to trap Ganon in Hyrule Castle. Link is brought to the Shrine of Resurrection to be restored, and Zelda brings the Master Sword to the Deku Tree for safekeeping once Link re-awakens.
Also, something else I noticed that I wanted to point out... During the Switch's Treehouse presentation of the game, there's an NPC they talked to that wants Link to tame this white horse and bring it back. He specifically mentions that he believes it to be descended from one of the royal family's horses because they had a pure white horse, and his grandfather would see them take it to graze at the same location where this one lives. And we've seen a pure white horse once before.