And Blizzard approved it, if not requested it. It's not like they said "hey, write a book about Arthas for us", she came back with that and Blizzard went "oh FUCK I guess it's canon now, damn it" then hired her to write another five books.
Everything Golden writes is approved by numerous writers at Blizzard, and they constantly make suggestions and requests for things to be included. The most we've gotten out of Golden is Anduin having a bigger role in Mists of Pandaria, and arguably the rest of his arc.