If Zack wasn't tagged, the question would seem more reasonable... though the answer is easy.
Yamatai is likely to use its own equipment, but where permitted, individual soldiers can use equipment according to their own choice, and it's reasonable that a Nepleslian serving in the Star Army would use NAM grenades where convenient, if permitted to do so, if supplied with them for whatever reason (including their own purchases).
That was easier than trying to counter-criticize him.