The entire argument is one of Relativism, there is a whole lot of disciplines where there is a huge subjective element, consider journalism and literature in general, but that doesn't mean we can not have a conversation about quality of work in these fields.
Software is no different. So when someone claims that "you can't question the quality of AI output because quality of software is [partly] subjective", they're appealing to subjectivism. This position is generally held by the incompetent to entirely dismiss any question of craftsmanship or quality.
No, you're missing the point. I'm not saying software quality is relative, I'm saying that most current debate on what people think is quality software, is subjective nonsense.
I've also never claimed that "you can't question the quality of AI output because quality of software is [partly] subjective". I'm not sure how you got there. I am simply saying that "quality" is an ill defined term and this is why we might see so much divergence when discussing AI code "quality".
You can absolutely evaluate code quality objectively, e.g performance, bugs, crashes.