I can't actually see what Jack's point is.

Amergin doesn't say anything about the Guardsmen in general, the song is about one particular man remembering what he as an individual did all those years ago.

Amergin's is speaking in the imagined voice of one National Guardsman who did fire into the crowd. "We were firing", yes - but "they all agreed the man with the gun looked a lot like me "; and "a shot rang out, I saw the first one die. I said not a word, but fired into the crowd "

Whatever may have happened with shots in the air or whatever, there was at the very least one National Guardsman who did precisely that. In fact, if there had been only one guardsman who did the killing that would make the idea of the song, of an individual living with the memory of what he had done, even stronger.