His experience pre-dates the huge increase in private trading in the late 1990's. I don't say good trading rules should have come out of the frenzied daytrading of the tech bubble, those were atypical market conditions, but some things in the game have changed since then.

His experience was (I think) derived from going long on US stocks in the late stages of a decades-long bull stock market and US economic expansion. As far as the markets were concerned then, I'm sure the feeling was this was never going to end. But being a long-only one-country share-buyer probably doesn't count as trading as we now understand it. How relevant is buying shares in a bull market to trading both ways on the huge variety of other instruments we can now access?

Sorry if I've taken a swing at your personal guru, but I suspect those of us who know the rule also break it every trade. Don't you?