What did they mean by this?
It means you gotta have a good meal anon
Translation summary: Atomic Samurai couldn't even see King's motion when he cut the apple. Even the apple hasn't realized he's been cut yet. Atomic Samurai can avoid slicing body parts and blood vessels, but to the extent of avoiding to the atoms is an impossible feat...hence why he is beyond impressed.
"Hahaha! I've been done for!" Atomic Samurai then tells his disciples he's going training from the beginning, and asks if they will join him