Somehow you managed to miss the point entirely. The point is not "use bad grammar, slang, and swear words." It's just "don't put on airs!" That's it! Not putting on airs means: don't use jargon if the meaning of your sentence doesn't call for it, and don't use awkward turns of phrase to make yourself sound smarter. That's easier said than done, but it goes a long way toward making your writing easy to understand.