Interesting article. Cheers, Shang. The only bit at which I pause critically is where it says:

For instance, for most Southerners "I might could leave work early today" is a grammatically acceptable sentence. It translates roughly as "I might be able to leave work early," but might could conveys a greater sense of tentativeness than might be able does. The use of multiple modals provides Southerners with a politeness strategy not available in other regional dialects.

This is only a "politeness strategy" if they routinely use "might be able" alongside "might could" in the relative terms described. Equally, it is only more tentative *if*... Otherwise "MC" is either a dialectal replacement or synonym for "MBA". The article implies the former, but doesn't actually state that it is so, nor give any evidence.