Thanks for clarifying.

What I meant was that there was no themed armour in The Labyrinth of the Ancients (themed, as in themed after the bosses), therefore the naming of Phlegethon's X could just denote equipment that is fashioned after the final boss of the Labyrinth of the Ancients. Rather than replacing the loot in The Labyrinth of the Ancients, they decided to put it in Syrcus Tower.