The reveal of the emperor's identity just felt like larian pulled it from their nether regions. I'm positive the only reason they did it was for the 'wow' factor of the name recognition, because it adds nothing to his character, and nothing to the plot. Same as if the character was revealed to be 'actually Manshoon' or 'actually Anomen' the reveal feels like a very thinly veiled attempt to assign importance to a character by taking the identity of another known one that never needed to be anything other than a normal mindflayer in the first place, and who could (should) be important to the player through their personality and actions within the game of BG 3 itself. It was tacked-on, in essence.

It's an irritating trait of Larian's writing in BG 3 IMHO that they need to make every major npc (particularly party members though) have this ridiculously OTT importance assigned to them in the world.

Emperor never 'needed' to be anything other than a mindflayer who had broke free of the Elder Brain's control and was pursuing his own agenda