The banners, the statues and the lot are references to Akatosh, because the Septim dynasty are related to Akatosh.
You can not hide your head under a bush and say that nobody can not see you, the whole trailer is about the dragons and their return, not Akaviri invasion.
You totally missed the point of my post: "They" in the trailer can't be dragons, because their existence can't be disputed by inhabitants of the Empire. Dragons served in Tiber Septim's army, it is an historical fact. Doesn't mean the game won't have dragons, it just mean that the "invading" force can't be just dragons, because people believe they existed. You can't ignore the Elder Srolls lore about dragons, just because you don't want to see anything else.
And some correction to what you claim:
The Imperial banner is Akaviri (Tsaesci) inspired (like 99% of the Empire from the Fighters Guild to the Blades going by the Legion and the armors/weapons designs) and Tiber Septim isn't related to Akatosh, he actually got stronger ties to Lorkhan/Shor anyway and was a Nord/Breton. The Nords don't like Akatosh and some of them actually want him out of the pantheon.
Also, if you go with just plain dragons, I would like to note that Aka mean dragon and vir mean land. Or if you need help reading: Akavir = Dragonland.
As for what I believe the game is about, I think "They" is the Underking. He don't have much to do with dragons, beside one of his "personalities" being called Ysmir Kingmaker, the Dragon of the North. But I do think that the Akaviri just make nice fodders, like the Mythic Dawns cultists did in Oblivion and the Six House was in Morrowind.