The Chaos Warriors come out of warp storms, are significantly stronger than normal humans, have better armour than the other humans, and, in old versions of the fluff at least, could carry plasma pistols. How exactly are they not chaos space marines?
My own take on tying in Sigmar with the Primarchs would be that he found his pod, went back to the Emperor, and fought in the Great Crusade. Sigmar wasn't worshipped as a god until a couple of hundred years after his disappearance. He was just an heroic leader and founder of the state (but not a dynasty... why didn't Signmar have kids, hint hint, Space Marine genetics anyone?), so there's no need for the Emperor to get all hoity-toity. I don't know why the theory has to be even complicated, let alone difficult to believe, it seems pretty straightforward that if you want, Sigmar can be a primarch, and Warhammer World can be an intimate part of the 40k Galaxy (as it used to be).
However, the main reason at the moment against that interpretation is *GW say it ain't so*. So it ain't, any more than saying "The Emperor is Superman" or "Leman Russ is Gandalf" or "Horus is Peter Rabbit" makes it so. Officially, at the moment, there's no connection between 40k and WHFB.
Not that I think it's daft or unworkable or anything of the kind. I'm pretty certain that there was at least the idea in the heads of GW in the late 80 and 90s that Sigmar might be revealed as a Primarch, at the time when the one system was just supposed to be the extension of the other. But the systems, and their background, have diverged somewhat over the years.
:speculative antiquary cyclops:
"Well it's Forty-one Thousand Nine Hundred Sixty-nine OK -
Gotta war across the Milky Way - "
Iggius Popiscus and the Stoogii, "41,969"