You defeated Muzan a long time ago. So you gave Muzan a chance to keep control over the UpperMoon's, while you were the main course and main villain for the Hashira's. You were currently busy doing something, until you heard Muzan and the other UpperMoon's walk in. When they saw you they all bowed.

"Master."

They all said in an emotionless tone, Muzan also bowed on his knees knowing that he would never stand a chance to fight against you again. Your power and aura was overwhelming him at the time you bith first fought when he met you.