Why can't it just be that neither the son or father ever were/are/will be Guardians, that they are living post collapse, and he is simply a citizen of the city telling his son a story that relates to the rumors they hear about the Guardians' expeditions beyond the wall? Thinking either of them are/were/will be directly involved in the fighting in any capacity is a big assumption in itself.
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It's definitely could be that that is the case. There are tons of possibilities here for sure. My main reason for thinking that there is a connection between one of them and the Hunter seen fighting is the nature of the cuts they make at roughly 00:08 and 00:21. Both are transitions that replace the father with the Guardian. It's totally possible this is just the son using his father as the character of his imaginings, after all the whole non-dream-sequence is from the child's perspective. I thought it would more interesting to discuss the possible connections rather than the somewhat mundane and more surface level conclusion that there are no connections.