That's been canon since at least the Sin War novels which started in 2006.
See, this is something I wish Blizzard would stop doing.
Okay, side-stories and such in novels is OK, but when you explain core concepts of a franchise only in novels to the point that someone playing all three games like I did who still doesn't know WTF is going on, that's when you're putting too much lore in novels and not enough in the games.
You don't know anything about "Sanctuary", or Angels and Demons doing it, or any of that crap until you get to Act 4 Diablo 3 from what I hear.
So basically, you play Diablo 1, Diablo 2, and half of Diablo 3 thinking that "Angels" and "Demons" are close to the same "Angels" and "Demons" that you already know from other sources and then WHAM, Act 4 says "oh, no, we're not talking about THOSE Angels and Demons!" other than a few very vague hints in the previous two games that could easily be taken the wrong way.
I played Diablo 1 very thoroughly, as well as Diablo 2.
But, because I didn't buy a bunch of novels that I didn't even know existed, I was utterly clueless to the real story behind the games.
This is a mistake, IMO. Like I said, side-stories and filler is OK in novels, but not the core concepts behind a story. That's like, oh, I don't know, releasing a movie and a couple novels. You watch the movie and you're going "Errr, wtf?" because you missed out on a lot of "this is who this is, this is who that is, and this is why they're fighting" that the books tell you.
They have been getting very close to doing this in WoW, too; I notice some of the newer novels explained a LOT that isn't even mentioned in-game (like Alexstrasza after the events of Wrath, the whole part where she loses her mate). So a player who only plays WoW is scratching their head when they're talking about eggs getting corrupted and such and they're going "I thought we saved those in RS?" but yet they didn't know otherwise because they didn't read that novel.