I don't think that's Alistair. Mostly because, well, what happens if Alistair is King? Or drunk? Or dead? Who would stand in his place? Especially for what seems to be a very important scene/boss battle.
Also, that appears to be rogue armour. Wouldn't Alistair be wearing heavier or bulkier armour because he's a warrior?
Good point about the armor.
But just in this particular playthrough he could be a Warden. Maybe in other playthroughs, if the Warden is still dead, he shows up as king. If he's dead, then the Warden won't be, so. Warden would show up.
But agree with Obito could be Orlesian Warden, because:
.@Dasieva426 That is not the Hero of Ferelden / the main PC from Origins.