That narrative comes from the military "investigation" and I don't think I would fail to recognize my brother, so meh. Military cover-ups are far from rare so I certainly don't discount that possibility. It is definitely a possibility, but far from certain imho.
It's also entirely possible that the life of a vietnamese peasant is so simplistic, or just different, that after 40 plus years he just doesn't remember that much of what he left behind.
It's not unheard for for people to forget how to speak their original language. There are cases of this happening after just a few years. So that doesn't mean anything.