They were partially right! The moment they used such techniques, the dark master leaders panicked and felt great threats. It wasn't because many of the scary monsters had gotten killed or wounded during just this wave of attack, but because this attack came at the same time as William's retreat.
All of sudden, everything they thought so far was wrong. These masters didn't come here to infiltrate the zone, but to send that gold master to do these weird things. And now they were going all out to help him escape.
Yet when they thought about their options and put aside sending any of them out there to stop William or the other masters, they realised they had nothing to do to stop anyone!
The high-end monsters were already fighting against Sloth and his team for many days now. They were close to the edge of the Black Holes zone, but that wasn't considered an advantage at all.