Why does Invincible Conquest state the Paladin can make one additional attack when taking Attack action?

@JeremyECrawford In Xanathar's Oath of Conquest, why does Invincible Conquest state the 20th level Paladin can make one additional attack when taking Attack action? Does it stack on top of their existing extra attack or in place of it?

— Ryverwynd (@Ryverwynd) November 22, 2017

Invincible Conqueror says you can make an additional attack when you take the Attack action, and it means exactly that: an additional attack. The feature doesn't care what else you're doing with that action. #DnD https://t.co/PGhlw6U27j

— Jeremy Crawford (@JeremyECrawford) November 22, 2017