I always read that scene as Sukuna just taking out Shigemo for being one annoying ass fucker.
Sukuna doesn't need his hands to slash someone - see how he cuts Yorozu back in the Heian Era.
Look at the exposition here.
"Until Malevolent Shrine is gone, it will relentlessly attack all targets within the effective range of it's guaranteed hit"
So the moment it stopped slashing Mahoraga, imo was when MS went away.