Targon? Space-farers?

From the understanding we currently have the original Targonians were a people who existed before modern human societies on Runeterra, are responsible for the peculiar architecture on Mount Targon, and left Runeterra a long time ago as they went to colonise more planets. Diana, Leona, Atreus, Taric(soon) and all previous Aspects are all empowered by way of having a Targonian basically inhabit their bodies.

Whether the original Targonians were humans or some other species before they advanced in to a space people and developed their technology/magic such that they could become celestially aligned beings is unknown, though I'm entertaining the headcanon that the darkin were part of the Targonian race at some point.

This is what a Rioter had to say about the actual Targonians In the Aurelion Sol Q&A:

The people of Mount Targon -- like the Rakkor, for instance -- do not travel to space in starships or anything. They are human and bound to this world in the same manner as other factions.

The Aspects, however, are more like spiritual entities that exist out of time, potentially in other planes of existence. They multiple “possess” hosts on different worlds and use them as their avatars. They don’t have spaceships, so swords and spears (and firearms) are still the de-facto weaponry on each world.

So the Natives of Mt.Targon aren't a space-fairing (body snatching) empire.

The celestial aspects are.