That scan that was taken from Thor # 493 by Warren Ellis does not denote Marvel's official position on the nature of the Asgardians existence or lifespan. At most, that scan depicts writter Warren Ellis' ill-informed position, as there are decades worth of Thor continuity that more than solidified his place as an actual god. That comic is simply inconsistent.
An alien, or "extra terrestrial", generally refers to a natural born biological being from another planet in our universe apart from Earth.
The Asgardian race predates the universe. As depicted in Thor # 85, as well as several other Thor comics, Odin created the world as we know it. His lifespan has already exceed BILLIONS of years, considering he presided over the creation of Mjolnir which KILLED THE DINOSAURS.
These beings are not aliens from another planet, they're entities that exist in a dimension above our own, and created the world, including human beings. In short: They Are Gods.
Marvel makes a big difference betweenn the scientifically explainable aliens of thier universe and the gods and demons that are of divine/magical energy and origin.