If you have year-round, full-time contract work with no downtime due to acquiring new clients or negotiating new jobs with existing ones, then you probably don't have enough clients to be truly independent.
Well, yes, but you can change the numbers to account for that. Say, bill at $100/hr? Or take the route of selling software. My main point was that the bar is much lower than the targets he gives, if your goal is "make a good living" rather than "become rich".
I'm sure there are unforeseen expenses in all of those scenarios that would require some padding to insure against, but the contracting scenario is the one I'm most experienced with, and the assumptions behind your numbers have bitten me in the past.