I was just confused because you have previously defined "core GW fluff" differently.
Well, the old articles on the DW we have are the most solid sources. I wish they'd bring out a new Codex:Deathwatch, I don't quite understand why they have never bothered to do that. GW should really support the most successful of the licensed products more. Blood Ravens have a lot of fans still and while I don't fancy them too much myself, I don't get why they don't get some official rules.
Anyway, a C:D might provide some clarity and could be fun to play. Poor Xenos players though - but given the current Taudar, etc prevalence, it might restore some balance.
The "wiggle room" you allude to has always seemed to be primarily concerned with tone and atmosphere rather than detail - although there are a some very few things that need to be universally consistent, such as the Ultramarines wearing blue. Gav Thorpe talked a bit about this concept on his blog . Everything else? Possibly true. Or just a myth/legend/propaganda/hearsay, depending on how you want to perceive it and to which vision you want to subscribe.
Interesting blog entry. It pretty much confirms my impressions of how 40K works.
Alex