I love it how Celestia, almost absent-mindedly, basically said “Luna told me about it” there, even though she heard firsthoof about the Mane Six having become evil when Applebloom, Scootaloo, Sweetie Belle and Spike sent a letter to them.  
It shows how much confused she was there in this moment, after all that happened, and how everything had taken a toll on her.  
It doesn’t matter if you watch an episode or read a comic issue, the writers always get those little touches right.

Why does continuity always seems to be just acknowledgement?
 
And, why do they bring this up?

I miss the days when the credit page had original artwork.

Either she was offscreen when this happened or one of them told her about it.

Cannot wait for the foul-mouthed edit. X3