@orion parish  
@sTs
 
Personally, based on how quickly Celestia knew and got to Ponyville and the fact that personifying itself is apparently a new skill for the Tree, I think it likeliest that that was the actual Celestia in there – but also that something else was at play in order to get Celestia in the… astral plane or wherever it is and to perhaps help catalyze Twilight’s ascension as well.
 
I found it a bit odd that apparently Princess Celestia was able to just pop in the princess making dimension without the episode providing any explanation why she was able to do so or how she knew to be there at that specific time – or how one travels there at all, or what “there” even is, and a lot of other things if I’m being honest – and I figure that Tree might be as a good an agent as any to get her in and out again with proper timing.

@orion parish  
Maybe the tree was a part of it as well?

@sTs  
Interesting idea for sure. But then how would Celestia have known about Twilight being an alicorn that fast after coming back to Ponyville?

Already I’m thinking that it may not be Celestia.