Probably ordinary, rank and file militants of the Purifier movement would have fled. But these were handpicked men, who were perfectly willing to die for their cause. With the grenades launched, they'd open fire with a round of fragmentation grenades before activating their flamethrowers, so that their genejoke enemies could recieve the purifying benediction of holy flame.

They would back off as needed to avoid being sliced and diced, but they weren't about to break and run in the face of mutant opposition.