I disagree with that advice, as you can't foretell which schools will split up the application so some sections are evaluated by different screeners. As every school doesn't have the same person read both the PS and the Activities section, I feel each needs to stand alone. Try not to be repetitious and perhaps try to vary the emphasis in each place: use varied vocabulary, maybe add new anecdotes, put insights, lessons learned, and future direction into a MM essay, and description, role, and results in Activities, or a different mix as suits you. The PS can possibly use another anecdote, and give an overview of the highlights, but described in a different way.