No, not everyone knows this. In fact, nearly every public speaker is initially taught not to do this. (I was in Forensics in high school, and recently joined Toastmasters.) For most people, if you try and memorize a speech, it will look like you've memorized a speech, because you will be fumbling for where you are next.

If you are a professional speaker on a tour, someone with a lot of drama experience, or a salesperson with the same pitch every time, then it might make sense to do Gladwell's approach. For the rest of us, practice extensively but don't memorize start-to-finish.