These are intended to be 'Human Rebels' - that is, non-Imperial humans, defined as rebels by the Imperium. In the fluff for 40K, there's far too much focus (IMHO) on the Imperium and all fluff seems to be told from their perspective. These chaps are intended to represent 'ordinary' humans - not necessarily Chaotic, just independent. The Imperium's far too big (and busy) to have an established presence on every world. I like to think that the majority of worlds marked on the maps as 'pro-Imperial' are in fact planets where a ship's company landed about 500 years ago, the lieutenant marched to the nearest village and announced to the nearest peasant that they now lived on an Imperial world, and then promptly departed. On lot of 'Imperial' worlds the citizens don't even know they're part of it, and on the ones that do, the Imperial presence is long-since scaled-down and faded into an almost mythical tale.
So when the Imperium comes back six millenia later and everyone's forgotten them, they get a bit shirty when the occupants of the planet protest 'independence' - hence this militia of independent humans. They are still equipped with bits of Imperial gear (among other things), because modelling a whole army of people without using any Imperial weapons would be a bit impossible. No real plans for these guys, I just dug out the models and thought they looked okay. Enjoy.
Thanks for reading!