Well, lore-wise, Aotius and Mutios are godlike beings that can create "aspects" of themselves in the form of various legendary Pokemon. So it's not impossible that Actan and Lanthan are aspects of them, too; pretty much every legendary duo has some part of their light. Think of them like unknowable cosmic forces: sometimes they can take Pokemon shape, but that's not all that they are. These designs are the closest we'll get to seeing their 'actual' forms, but even these are just a representation.
They do have Pokedex numbers though so I figured they deserved having sprites, too. I'd always planned to add them to the game eventually. In fact they are the earliest legendaries ever conceived of for Uranium, ~JV~ came up with their names back in the early stages of the game.