Not all factionless are artifacts, and we were told to consider factionless like MTG's Artifacts for ease of reference.
Factionless people could come from any and every world. Take Genesis Industries for example. They may be the biggest faction on that world, but it doesnt mean everyone in it is part of that faction. So Conscripted Militia could be a bunch of people who have teamed up with Genesis Industries (in your deck) because without them, they would just be tiny little bugs to be stepped on by a Skraar.