but spelling and grammar still isn't a good indicator for expertise, intelligence or anything like that even in an academic context
Mainly:
1. Dyslexia doesn't make you dump, just likely to misspell and a less likely to notice your misspelling.
2. When speaking about neurodivergence people mainly think about Autism or ADHD but sometimes just mean that your brain thinks in very different patters, this can make grammar hard. Especially if it's not your native language.
3. Sometimes people had shitty situations earlier in their live, leading to incorrectly learning parts of languages. This is hard to fix. But isn't really representative in any way for their expertise in any topic which isn't the given languages grammar.
4. English grammar and pronunciation to spelling mapping aren't exactly well designed. People not wanting to bother with it is not really related to intelligence, or excellence in other topics.
5. Some kinds of expertise are unrelated to general intelligence, expertise, education. So even if spelling and grammar where related to intelligence, it wouldn't be meaningful to judge expertise.