Is that how native speakers understand this proverb? I always assumed "a good man" here is synonymous with "a good human", because that's how if goes in my language, and because it's the only way this makes sense.
I think it’s because “a good man” is quantified by the indefinite article “a”. Whereas “man” in a sentence without that qualification, e.g. “one small step for man” I would not interpret the same way. So, “a man” = a male human, “man” = maybe a male human, maybe any human.