It’s a matter of skill and potential. Yorna is an ancient monster girl and while she hasn’t returned to her full power at the moment of the boss fight, she’s still very strong. To effectively work, silencing user must be at least neat target’s power level. Otherwise, the target will hardly notice the hindrance.
Imagine you are playing tug-of-war with a first-grader. Most likely he won’t be able to do anything. Swap the child with a pro powerlifter and suddenly the situation changes dramatically.