The Jahal mercenaries for their credit did not break rank and run after facing Grahtos.
Instead, when they saw they had no way to counter the new weapon and their center was vulnerable, they deemed it unwise to continue to fight here and needlessly rack up casualties.
So they simply chose to disengage and retreat.
And the difference between a retreat and a rout was in full display them.
For while a rout was a disorganized and chaotic escape from the battlefield, a retreat was an orderly withdrawal, as evidenced by how the various units calmly turned left or right in full formation, so as to not give Grahtos any chance to hunt them down, before getting out of Grahtos's strike range.
And seeing them scatter, Grahtos let it happen without interference, for he had no intention to chase such a huge enemy anyway.
He was just relieved to chase them off and secure his left flank.