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Everything is true, nothing is canon.
The only canon you can rely on is the latest edition of a rulebook, codex, or campaign book. Black Library is optional.
Is it set around an important 40k character such as Macharius or Marneus Calgar? Most likely to be considered canon.
Is it set during an important event such as The War of the Beast or the war for Damocles? Again, likely canon.
Is it a Guard regiment in a far-off sector doing heroic deeds i.e. the Tanith? It's only canon when it's in a codex, which thankfully they are.
Is it set in a video game within the universe? You're pushing your luck and once again only if it's featured in a codex i.e. the Blood Ravens.

If it's a codex/campaign book/BRB associated with the latest edition of a game then that's canon, that's how GW operates. If the current books contain stories from older edition codex/campaign book/BRB then those stories are canon.
For example the 13th Black Crusade. GW had it as an ongoing end-of-days conflict, then did a worldwide campaign that didn't give them the result they wanted so it was removed from canon the next edition of the game and went back to unending conflict. Then Gathering Storm brought in new canon that showed the definitive end to the 13th Black Crusade and the beginning of the Dark Imperium setting.
My IRL example of canon-conflict was when the Blood Raven Librarian was released as part of the Deathwatch: Overkill box and a GW staffer said that was the Ravens now a canon chapter as they had been featured in mainline GW product despite the many Dawn of War games up to that point indicating they existed. But because Dawn of War was not produced in house by GW it wasn't considered canon and they very cleverly side-step the whole issue of "multiple video game endings" by having the conflicts of those games a closely guarded chapter secret, which was printed in the Index Astartes in White Dwarf.

That's what I've been told by people who work for GW and sell its products. Whenever we discussed background in-store the final ruling was "if it isn't current it's not part of the canon". There are sections of White Dwarf written by the design team that is fully dedicated to saying what the universe is and what is currently canon.
Plus, your opinion on BL publication has absolutely no bearing on whether they are considered canon or not.


He's the creator so yeah credit, where credit is due(Thanks for starting the hobby that's taking 60% of my monthly income Rick) but not part of the current writing team or owner of the brand so anything he thinks should or shouldn't be canon, is a moot point.

For the most part, I would say GW doesn't make huge changes to "historical" events unless those events consist of "there was a big fight the end". Like the Horus Heresy and Macharian Crusade were very empty but now we have loads of lore to discuss after the huge boom in content the Heresy got and the William King novels for Macharius. Even stuff around some major characters, like where Calgar supposedly holds a city gate for 10 days by himself but it turns out a chunk of 1st Company marines died alongside him, which I believe was a recent addition?

Priestly's opinion about canon is moot because he left GW in 2009. He isn't involved in game development with the setting so it doesn't matter what he says should be canon or not. Technically his opinion means as much as yours or mine because he isn't making the business decisions that drive the background of the game.
I can say all I want that Ahriman was loyal to the Emperor because Rogue Trader says so and I don't consider new material canon. That doesn't matter because GW has Ahriman firmly in the "I'm a huge traitor that hates the Imperium" camp.