Twain is of course being satirical here, but I can tell that many people have an overly strict approach towards language learning. They expect rigid rules and get annoyed when the language does not adhere to them, yet they do not realize that these rules came after the language and they are most often a tool to teach and analyze it. What language instruction is supposed to achieve is providing one with a foundational understanding of language, just enough that immersion learning becomes possible. Since human language is a mix of logical thinking and fuzzy pattern matching, there is no other way to learn it completely than by pattern matching itself.