I think he's assuming that if you retire at 65, you're living off of SS (from 40 years of working, the most lucrative of which 35 count) plus your own savings. If you retire at 55, you have to cover 10 years of expenses all on your own, and *then* use your remaining savings, along with a lower SS check (since it's only 30 years of income that counts), for the remainder.

How someone is expected to save up 33 years of *salary* in 30 years appears to be left as an exercise to the reader.

...which I did. :) Assuming 7% returns and a constant salary, if you save 35% of your salary each year, and start at age 25, you'll have 33x your annual salary when you hit 55. Of course, lots of people are expecting lower than 7% returns, and your salary usually starts lower and peaks at the end of your career, but a 3% withdrawal rate is pretty conservative, too :)