I didn't find a blue post, but obviously the Celestials on Timeless Island have been greatly buffed. Their health went up from 75m to 370m, and their abilities do significantly more damage, i've seen tanks go from 100% to 0% (probably some tank swap mechanism that could be ignored before).
While I agree they were greatly undertuned before, now they pose a real challenge, especially of you play on a realm/faction that won't easily gather 40 players. Of course they will get easier from week to week, but right now they seem to be quite hard.
In my opinion they should have made them scale with the players attacking, like iot rares.