An episode that thoroughly builds upon the themes of the story and has an effect on the main characters, but isn't directly relevant to the main scheme of the villain? Filler.
An episode that expands upon the characterisation of multiple characters and the relationships between them? Filler. I've seen people call ATLA's Tales of BaSing Se filler, even.
I guess it's filer in the sense that it doesn't advance the main storyline of ATLA but it sets the stage for what is going to be a rather dark and serious denouement. And it's probably one of the most important glimpses into Iroh's story and motivations. I can't imagine ATLA without that episode.
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u/OK_x86 11d ago
No. That story while horrific does set the tone for some of the war crimes state alchemists pull at the behest of the Fuhrer.
It's not filler at all.