Let me spell it out: that's the defining moment of Tank Man's life, you have zero clue about what happened prior to that moment. And that's because really nobody seems to have a clue about who he really was, what his life was like up to that moment and so on. It's a singular act of bravery and one that I very much feel moved by. But I have zero illusion that Tank Man's life was nothing but an endless succession of moments like these where at every opportunity he did the right thing. And that's perfectly normal, doesn't make him any less, in fact it makes him more: because ordinary people making an actually stand when it matters is very powerful, no matter what their lives were like up to that point.

What point are you trying to make, actually? Your argument seems to be that "authentic humans," however that's defined, are not just rare but nonexistent, even if other quietly heroic people who fill the same role do exist. I'm not sure what end you're trying to reach here.

I'm far more interested in the interplay between people and their circumstances than I am in labeling a particular human as authentic or not, it smacks of a value judgment in a vacuum, whereas IRL there is no such thing.

Your problem is that you stated such people don't exist despite the vast historical evidence to the contrary. Hitler's own officers wanted to kill him. Think about Harriet Tubman, or the underground railway. ad nauseum.

Character is important.

As you say we don't know much about Tank Man. It was probably more than the defining moment of his life, it was the last day of it.

Isn't it fair to assume that whatever led up to that moment was a life lived in good character? He may not have lived as an exemplar of "authenticity", and if it was only a moment in an otherwise selfish and wasted life, like those "city businessmen" who spontaneously jump onto a subway track to save a child, we agree it doesn't that subtract from the authenticity of the act?

As we say around here "carefully pick your hill to die on". But there's more to the picking than a rash decision in a single moment. There's everything that built that character too, no?

So let's not resort to theoretical absolutes. I don't know where 'here' is but every culture has similar idioms but at the same time every culture has something about being careful about meeting your heroes.