Noah's attack didn't have the appearance of a slash. He had attacked with his blade, but the area covered by the singularity was too big to be part of sword arts.
Those effects came from the different powers deployed by Noah during the attack. His creation and destruction generated singularities when they clashed, and his dark matter increased the size of his offensive in ways that normal "Breath" couldn't achieve.
His sharpness had reached a superior level due to the many features carried by his law. It had transformed into singularities capable of piercing everything in the world. Even the air in the sky vanished under that power.
Noah inspected the area of nothing generated after his attack, and his eyes sharpened when he saw something moving inside it. There was a slash flying through the singularity.
'How is this even possible?' Noah wondered. 'He is even limiting the power expressed by the attack to the gaseous stage!'