Yes there is a possibility that things can break like connecting rods etc. and do more damage. Allowing the engine to continue to knock (assuming it is a thrown Rod) will only trash an already trashed bearing and crank journal. DO NOT UNDER ANY CIRCUMSTANCE REV OR GUN THE ENGINE. Idle it only.
If you start the truck and let it idle while you pour in the cleaner, it is not likely. If your problem is lifter knock, the flush technique should solve the problem fairly quickly. If your problem is a thrown Rod or Main bearing, the engine flush won't help and the few minutes it will run probably won't make a whole lot of difference.
Does the knock go away when the engine gets warm? Mine went away after a few minutes of warm up.