Congratulations, you've sufficiently annoyed me enough to log in to my local instances to type this out.

There is no ”one” way to speak and write English — we don't have an """official""" institute of our language like Spanish or French does (and even if we did, they would not have a monopoly on English). We don't speak in Received Pronunciation or keep the superfluous 'u' next to every 'o.'
Like every language, English has multiple dialects with their own vocabulary, and even some with their own specific grammar. The sentence in the OP was likely written in one of them - African-American Vernacular English. This dialect codifies double negatives, the habitual be, and words like 'finna.' Many of its aspects are already integrated into 'standard' American English.

This is part of the process of language in general. Many of the rules in 'proper form' come from shorthand, slang, and and crude versions of other languages and forms. Being aware of the rules shifting and changing as people shift and change how they speak will probably get you further than turning your nose up at rules you don't recognize.