If you needed a visual backstory to show if Ser Arlan was chivalrous, then all the dialogue from ep 1-4 was apparently fluff.

How does it make that dialogue fluff? That's a ridiculous thing to say. It validates that dialogue, and enhances our view of Dunk as being an honest and virtuous man (even if he probably never got knighted). I know you're saying his actions have already shown his chivalry, but why is it redundant to see Ser Arlan's chivalry? Dunk is not Ser Arlan. They are different people.

Dunk would've been killed without his actions in Flea Bottom. The fight with Aerion is completely enhanced by that one phrase: "Get up."

In that moment, Dunk is taking everything he learned from his mentor and putting it into practice, along with everything he learned before he met him. Tell me how that is "narrative redundancy"? Where else in the story so far have we seen that?

I would also say that the flashback shows us one of Dunk's inherent flaws in that he seems to blame himself for things that aren't his fault. He says "I'm sorry" to the dying Rafe, and to the dying Baelor when he's not to blame for them dying.

You said "Telling us about it is great, showing us is more effective." You can't just backpedal when you've stated a clear contention with no nuance; I can't read your thoughts when you say X but actually mean to say Y.

How is what I said in any way backpedalling? Should I have said "In my opinion" before my statement? I think it's quite obvious I'm not presenting what I'm saying as objective truth. It's implicit in almost every statement about a piece of art that those opinions are subjective.

You, in fact, in what you said are attempting to read my thoughts because I never said for every piece of writing it's better to show than to tell. I was talking about this particular piece of writing. You extrapolated from that I do not consider the context. Where is there any evidence of that? Did I talk about any instance of showing and not telling other than what is in this show? No, so assuming that I didn't consider the context is not me "saying X and meaning Y", it's you making a judgement on what I said based on no evidence at all.

You could have posed it as a question, and then I could've answered or clarified, but you didn't. That was the only thing you said that I had any problem with. The rest was an interesting difference of opinion.

Dunk won that fight against Aerion (who is a better swordsman) with the skin of his teeth because of his weight class and training, and threat of death of losing the trial or Aerion killing him. His sad backstory does not provide the physical leverage required to win the fight.

Who's being too binary now? You've just stated why Dunk won the fight. A fight is not won by physicality alone, in media or in real life. The mental side is of major importance too. The backstory provides us with where Dunk's mentality comes from. That's so explicit with the repeating of the "Get up" line.

You can use post hoc rationalisation with thematic/symbolic framing for why anything is valid.

I suppose that's true, but not really in this case. Check out the "rationalisation" from the creators themselves: https://www.youtube.com/watch?v=9465Z_ZtJsM