Yes, that's a very good question as to why they wouldn't know. A question that is kind of brushed right the hell over. Again, the Amirdrassil epilogue seems to indicate that Thrall is not aware of the concept because he remains confounded by a female voice coming from a vision of a radiant orb. Which Azeroth would be at least a top 3 candidate on a list of suspects.
I really think they're just trying to make it a big deal that Azeroth herself is calling to heroes and the planet soul is in danger despite the fact that we've been aware since Chronicle.
Sargeras was going to stab in further and was stopped. A sword that big would've killed the planet. It's what he does. Period.