Dub, if thrusters are not an explosive exercise I'll eat my hat!
1) How you do the exercise. Thrusters is a long duration exercise, you have to do every rep explosively or there wont be any. Just doing them is not making it explosive. Think about doing squat jumps slowly. Does that make it explosive training?
2) 21 reps of anything is not explosive training. You can't produce maximal explosive force into the weigth for that long without your nervous system burning and muscles fatiqueing. If you can, the weigth is so low, you most likely aren't really developing maximal force. And when you are fatiqued, it's no longer maximal strength. Hence, it's not explosive. Sprinters don't do several 100 meter, even 50 meter dashes in a row if they want max speed and explosiveness. That's endurance.
For explosive exercises, reps shouldn't last but mere seconds and less. Explosivity is about rate of force development, or POWER. Explosive training and max strength training differ from the fact that explosive reps don't have enough time to produce absolute maximal force. Hence, they try to develop as much force as possible in that time frame.