Hearthstone is a game originally based in canon Warcraft universe but feeds upon multiple what if scenarios, which in turn makes it non-canon as a whole (or just those non-canon cards).
WoW is conceived as the primary canon source of lore. Heck, even some elements of some novels/comics can be considered non-canon.
Hearthstone canonically exists inside the Warcraft universe, but the things Hearthstone depicts do not represent the reality of that universe. Just like how WoW exists IRL, but its events do not represent reality IRL. This applies to Xal'atath. All of these things are in-universe fiction.
WoW is not the primary canon source. All sources are canonical except the tabletop RPG. Some parts of WoW are superseded by the novels/comics.