This has nothing to do with my post you quoted. You are referring to a later development. Grey Knights, Sisters of Battle, Assassins, Inquisitors, and Arbites all used to got in the same list of random units you could ally with your forces. That was their job, their purpose. At the tail end of 2nd the Sisters branch off. I think then it's Assassins and then you start seeing Grey Knights with power armor and storm bolters, that's when Sisters get rolled back in with inquisitors for reasons. If anything that wasn't a Grey Knight was added to the Daemon Hunters book it's because it was anemic. Nothing was shoe horned in and the only thing preventing it from being an update of Agents of the Imperium proper was the exclusion of Sisters and the fancy shotgun rules of the Arbites.
That exclusion would have sucked, but they put a TON of new stuff in the book. Death cult assassins, daemonhosts, henchmen of every make and model, power armoured Grey Knights (with a ton of new special rules). We even got options that weren't bolter-on-a-stick.