AYRT didn't say they don't take ANY input about how they write. They just said they don't take input about how they write their plots and characters. That still leaves quite a lot of area on which they will accept input.
We also don't know exactly what AYRT means by that. I mean, one form of advice might be: "The way this character goes from calm to sobbing sorrowfully in this scene feels very abrupt and a bit awkward. I think the emotions of the scene would land better if you incorporated his emotional transition a little more smoothly into the text." Is that character input, or something else? To me that feels more like input on the writing itself. As opposed to something like: "I think Character A is coming across too emotive and vulnerable, here especially, but also throughout the rest of the fic. I think the reader may find it hard to believe this is really him, as he's so much more stoic than this in canon." Here we have a piece of input that may well be correct, but if the writer wants Character A to be more emotive and vulnerable in their fic, then being told it's OOC to write him that way isn't going to help them any.
Of course, a really good beta is usually able to identify what the writer's intent is, and do their best to accommodate that (for example, accepting the OOC characterization if it's how the writer wants the character to be, while still smoothing out the writing so that the characterization - however OOC - remains stable, consistent, and relatively believable throughout). That said, a really good beta is - understandably - sometimes hard to find.
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We also don't know exactly what AYRT means by that. I mean, one form of advice might be: "The way this character goes from calm to sobbing sorrowfully in this scene feels very abrupt and a bit awkward. I think the emotions of the scene would land better if you incorporated his emotional transition a little more smoothly into the text." Is that character input, or something else? To me that feels more like input on the writing itself. As opposed to something like: "I think Character A is coming across too emotive and vulnerable, here especially, but also throughout the rest of the fic. I think the reader may find it hard to believe this is really him, as he's so much more stoic than this in canon." Here we have a piece of input that may well be correct, but if the writer wants Character A to be more emotive and vulnerable in their fic, then being told it's OOC to write him that way isn't going to help them any.
Of course, a really good beta is usually able to identify what the writer's intent is, and do their best to accommodate that (for example, accepting the OOC characterization if it's how the writer wants the character to be, while still smoothing out the writing so that the characterization - however OOC - remains stable, consistent, and relatively believable throughout). That said, a really good beta is - understandably - sometimes hard to find.