How is enjoying where he ended up in his redemption arc ignoring where he started?

We're talking about a redemption arc. That means The character started in a different place than they ended. How they ended up differed from where they began. It's in. It's in the discussion. It's inherent to the discussion.

"wow! Look at how far he's come" is implicitly a part of "Look at how amazing this character's redemption arc is".

Like no, of course Aa.ng yelling at Katara and Toph for things that aren't their fault and then never apologizing for it is not as bad on a consequential level as Zuko betraying Katara in COD. The difference is that he showed remorse and did everything he could to prove himself to her (and all of them) again, he is King of Giving Good Apologies.

The fact he shows such immense growth from where he started out to where he ended up is an indicator of what the rest of his life will look like. Based on the pattern we are shown, we can presume that he will have humility and remorse for future fuck-ups, and swallowing his pride to apologize for future fuck-ups should be very easy for him because he's already managed to accomplish the ultimate pride-swallow at an age where he was at his most emotionally immature and selfish (just scientifically, on a neurodevelopmental level, adolescence and teenagehood are the most self-centered and unempathetic a person will ever be outside of baby and toddlerhood). So he set a great precedent for future behavior and emotional maturity. Admitting you were wrong takes a lot of inner strength, and the more wrong you were the harder it is, so Zuko admitting to being so deeply wrong, admitting just how much he'd hurt other people, probably nothing else he ever does will ever be as difficult to admit to being wrong about.

Comparatively, Aa.ng shows very little emotional growth or ability to recognize his wrongs and apologize. And yeah he's 12 but he's also fictional, they could have written the other characters to stay a little miffed at him until he realizes his mistakes and apologizes to his friends, instead they are written to just seemingly forget about it and forgive him without him ever apologizing, which kind of makes it feel like they're not actually his friends but his allies who are tolerating his behavior for the sake of ending the war because he's the only one who can do it.

2) any vegetable is better than no vegetable