KOS wrote:Another idea is that the Grey Knights could be a first founding chapter.
And books are not canon stuff, so let's not think that they are completely right. Many times what is written in there, is not even close to what can be read in the various Codex that got out from GW since Rogue Trader.
Grey Knights were founded in secret after the Horus Heresy, and from what I understand, the Horus Heresy books have been given their blessing from Games Workshop, and just about everything in the Black Library (as I understand it) can also be considered canon.
As for Codices, they can actually often be considered as unreliable sources of information. They often go out of date, for one, and are also written with a slant, usually by the Imperials, or by the concerning race.