>>27079468
Keep in mind Warhammer and Warhammer 40,000 are worlds where half truths, lies, propaganda, politics, legends and myths exist. The absolute truth which is implied when you talk about "canonical background" will never be known because of this. Everything we know about these worlds is from the viewpoints of people in them which are as a result incomplete and even sometimes incorrect. The truth is mutable, debatable and lost as the victors write the history...
Here's our standard line: Yes it's all official, but remember that we're reporting back from a time where stories aren't always true, or at least 100% accurate. if it has the 40K logo on it, it exists in the 40K universe. Or it was a legend that may well have happened. Or a rumour that may or may not have any truth behind it.
Let's put it another way: anything with a 40K logo on it is as official as any Codex... and at least as crammed full of rumours, distorted legends and half-truths.
I think the real problem for me, and I speak for no other, is that the topic as a "big question" doesn't matter. It's all as true as everything else, and all just as false/half-remembered/sort-of-true. The answer you are seeking is "Yes and no" or perhaps "Sometimes". And for me, that's the end of it.