Mr. Mark has nailed it.
MMM manages to exploit a loophole in the system. You can earn enough, and save enough, if you frugal, to reach financial independence. With a relatively small stash, you can exploit the tricks built for the rich (low taxes on capital gains and dividends; investing in the stockmarket safely; preferential depreciation and tax treatment for rental properties, no wealth tax), and the tricks for the middle class (tax deductible IRAs, 401k, mortgage interest, property tax, health care, artificially low interest rates, subsidised public schooling and other services via location and housing costs), and the tricks for the low income (very low taxes on income, lots of free entertainment, public transport, goodwill stores, do-it-yourself, make do, practical solutions, low cashflow).
It's not a loophole, it's an understanding of the various pieces of the financial system and how to construct your financial universe out of those pieces. Consuming less than you make, investing the excess in real estate and tax deferred accounts, and enjoying what taxes make free or low cost to you are part of the plan.