Why am I saying this? How do you know they didn't hit TOGA a second time? Would make sense to me. If there are CBs about or WSHR it would make perfect sense. Notwithstanding the fact that pressing it once will give you a margin above buffet speed to account for any turbulence or shear surely it would make sense to use all power available. A windshear go around is an escape manoeuvre after all. I'd rather escape with a big margin than just what is required for certification. Wouldn't you?
Well, I don't know, that is why I told you what happens if you press TOGA a second time. It is not normal procedure as far as I know. You make it sound like a go around in a 737 is a near uncontrollable maneuvre if the aircraft is light. It is not.
The only reason you would need full go around thrust on a light 737 is if you are about to crash., or you need the performance for other reasons. Reactive WS happens below 1500 ft, so I don't think they entered that territory.
IF they pressed TOGA a second time, they could very well have made problems for themselves.
In a 737 you get what you ask for. If you don't know what you are doing, it will bite you.