Being well spoken is 100% of sign of intelligence. It's not foolproof, there are people at the margins, immigrants, unconventional thinkers, blablabla. At the population level all that fades away and you can bet money all day long that the people with the better grasp of language are the intelligent ones.
Sharp language is a symptom of sharp thinking. Pretending otherwise in a misguided attempt at inclusivity does everyone a disservice.
But people can be "well spoken" in their own dialect.
I'm in Newfoundland right now and the English here is WILDLY different. Someone here is not less intelligent or clever because they speak Newfie English. Many of them can also code switch depending on who they're talking to.
Someone speaking a different dialect is not making errors, they are following a completely different set of linguistics rules. They are perfectly capable of expressing themselves and their complex thoughts.
Assuming someone who speaks a different dialect is less intelligent is as rational as saying that someone who speaks a.different language is less intelligent.
Someone who speaks Acadian is not speaking bad french, they are speaking a unique dialect that blends English and French with its own distinct structure. It is COMPLETELY different from the blend of French and English that occurs in regions where both languages are spoken and where people substitute words for each.
I can speak French and English and can understand the Franglais blend that is spoken at the border of Ontario and Quebec, but it is not a dialect. There is no specific structure, just common errors and substitutions made on each side. Although I speak both languages, I cannot understand Acadian at all. I can pick up words here and there, but the structure is fundamentally distinct, so it's like a totally different language to me.
Rarer dialects have just as many rules and structures are more dominant dialects. They aren't just a collection of mistakes, they are fully structured and can be analyzed as such with syntactic analysis.
It doesn't sound like you've actually studied languages at all, meanwhile I've spent countless hours mapping out the syntactic structures of many languages and even more dialects of those languages. I've spent hundreds of hours parsing the rules and interactions, it's mind blowingly complex when you get down to the granular level of syntax trees and prime structures.
This isn't some woke nonsense, this is a formal, highly technical academic subject that many people have doctorates in and they literally pretty much ALL agree on these very basic, first year level linguistic concepts.
This isn't obscure social theory, this is intro to linguistics level information. Basic survey knowledge. Arguing this is like someone in intro to chemistry arguing against the concept of electrons acting like both a wave and a particle.