I couldn't really follow the conversation on that board.
It doesn't surprise me at all if a service member doesn't know the difference between different M-16/ AR-15 models or terminology. Not all of us are gun nuts. Most of them probably can't tell the difference between an M40 and M42 gas mask.
The A3 upper on an M-16 is the same as the A2 upper. The M16A3 is full auto instead of burst. The M16A4 fires in bursts, but has a flattop upper.
M4s with few exceptions that I've seen are flattop. The M4A1 is full auto.