The problem with the Sideways video is that although it's well-researched, it's presented in an incredibly skewed and disingenuous light designed to paint the situation as unflatteringly as possible, and it's very frustrating what a negative impact that video has had on the public's perception of the Avatar soundtrack.
Every movie in the history of ever went through a concept phase during pre-production. During the concept phase, the artists, writers, etc. working on the film explore hundreds of concepts, be it for story itself, character design, environment design, music and sound, camera work, visual style, AND MORE.
Only a fraction of what gets explored in the concept stage makes it into the film. For any film. Sometimes concepts that are really cool on their own are passed over because something else ultimately suits the project better, and that's ok. Sometimes concepts seem like they'd be really cool in theory, but then when but into practice they just don't work out as well as hoped.
And that's all that happened with the Avatar soundtrack. Cameron asked Horner to explore the concept of a strange "alien" sound, but ultimately decided that while it was a cool idea in theory, it just didn't quite end up working for what he had in mind in practice. And that's ok. It's not some great tragedy, nor "ironic", nor a "crime scene", nor "the musical equivalent of strip-mining" or whatever else Sideways claims. It's a very normal and expected part of filmmaking: explore lots of concepts, some end up fitting the project, others don't.
Sideways's insistence that "James Horner was robbed of his magnum opus" is also incredibly pretentious and presumptuous. Could this hypothetical alternate soundtrack have been an amazing feat of music way better than the score we got? Sure, it's certainly possible—but we don't and CAN'T know that for sure, not without having heard it for ourselves. For all we know, it also could've been a disorienting mess that distracted the audience from the story and broke their immersion. We just don't know.
If by chance you're interested in a much more thorough dissection of how and why the Sideways video is so skewed, including clips of several interviews from James Horner and Dr. Bryant themselves, and why the idea that the final score has "only echoes of their intentions" isn't really entirely true, I'd recommend taking a look at AnomalyInc's response to it in his absolutely massive Avatar deep-dive (the section on Sideways starts at 3:56:45):
https://www.youtube.com/watch?v=FMWx2AOGtQY
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