You say the book canon isn't separate, but whenever I look at WoW lore I see book-reliant information stuck in its own section with a little disclaimer so you know to take it less seriously.
To me it looks pretty similar to Star Trek or Magic the Gathering or any other franchise big enough to have mediocre books based on it - there isn't an official separation, but you have to take anything the books say with a grain of salt because the creators frequently ignore/contradict all but the biggest plot elements. That contributes to most people ignoring the books and two very different perceptions of canon forming. But I'll admit this might not be the case with WoW lore and I could just be misinterpreting how Blizzard and the fans handle it.
And the "two Hearthstone canons" comment was not serious.
The problem is that Blizzard doesn't do this, they've done it with Starcraft 2 where many of the newer characters comes from the Book Series (Explaining Tychus history with Raynor, Jimmy's Crew, some of the newer characters histories are
never explained ingame but they have been in the book series that came out beforehand). While characters in World of Warcraft have come from the book series (Rhonin was a prominent example) and major plot's have been resolved through the book series (Want to know how the whole Time Travel thing really went down with Garrosh for example? Was done in the books! Or how he convinced Grommash? In the books/comic!) The thing about Blizzard is that many of the writers of the books are those who also tend to work with Blizzards other works at times, so they can keep it more inhouse.
Also while Star Trek does as you say, MTG does keep it's books canon actually, they just don't tend to matter because of the medium too much. You probably should've stated "Star Wars and Star Trek's Expanded universe".
Not a big fan of this approach too much myself, sure it adds alot more detail, but I was literally blind as to who the heck the crew was in Starcraft 2 because there was barely any information on them ingame.