I am not a linguist, but I don't think that many linguists would agree with your assessment that dialects, leet speak, short form writing, slang, creoles, or vernaculars are necessarily ungrammatical.

From what I understand, the modern understanding is that these point to the failure of grammar as a prescriptive exercise ("This is how thou shalt speak"). Human speech is too complex for simple grammar rules to fully capture its variety. Strict grammar and lexical rules were always fantasies of the grammar teacher anyway.

See, for example, the following article on double negatives and African American Vernacular English: https://daily.jstor.org/black-english-matters/.