This seems to be based on a racist assumption. Why is speaking improper English labelled as “African American english”?. I would want to see the LLM assumptions also for southern drawl and for general incorrectly spelled / grammared speech, to compare to the assumptions made for the African American english version.

Speaking with slang / incorrect grammar is of course, in general, inversely correlated with education level and/or preference for shorthand forms of speech over writing/speaking the full grammatically correct form. The LLM is saying speaking in slang = stupid/lazy.

The researcher is labelling slang as specifically African American speak, therefore interpreting the LLM response as assuming African Americans are stupid/lazy.