He isn't saying everyone should use margin all the time but rather young people to "levelize" the difference between early portfolio and portfolio near retirement.
If you look at stock market over last 120 years the average return is 12% which is more than enough to retire on, however there is a lot of volatility in that average. The problem is some people get "lucky" and others don't depending on when they were born (and thus enter and leave market)
If you started investing in 1925 by the time of the crash in 1929 you wouldn't have that much money invested so in nominal terms you loss would be small. You could then go on to invest for next 30 years and have a good lifetime return.
However someone who invested in say 1890 and had had decades of good returns (on small amounts of money) would get anhilitated in 1929 on a large amount of money.
The problem is starting out the portfolio is small (say $10K) and near retirement the portfolio is large (say $1 million).
Years of nice gains on $10K can't offset even one large loss on $1 million.
Diversification helps but even the stock "portion" is unbalanced. Say you are 100% stocks early and 20% stocks at retirement.
Well 100% of $10K = $10K. 20% of $1 mil = $200K still a 20:1 ratio.
By using margin early on you can close that multiple. $10K margined to $20K early on vs $1 mil at retirement and slowly reducing margin and asset allocation could get that ratio down to 10:1.
Of course you need a broker who doesn't rob you on interest rates. Broker can borrow for insanely low rates (about 1% to 1.5% right now) most mark that up 300% to 500% for their retail clients.
If you have a broker who doesn't (1.7% margin rate on balances below $100K) it can work.
http://www.interactivebrokers.com/en/accounts/fees/interest.php