Disclaimer: I haven't read the book myself. But, my understanding of the basic premise is that an average earner can become a millionaire by living modestly, saving, and investing the savings.
It turns out that we have the perfect tool to test this: Firecalc! Even though I think it's over-optimistic, let's give it a shot. I ran it in reverse, starting with a 0 portfolio and using negative income, and it turns out that saving $15K/year for 30 years, invested in the default FC portfolio gives a 50% chance of saving about $1.2M. Now, the median household income in the US is $51K, and the taxes (SSI/Medicare + Fed) are about $6K, so that leaves about $45K for everything. So that leaves $30K/year for everything if you are going to become a millionaire in 30 years. And that assumes a 50% chance of "winning." If you want, say, a 75% chance it looks like you'd need to save $20K/year, leaving $25K/year.
No doubt there will be some here that say that they could raise a family on $25K/year, but I think most would agree that that would be pretty unpleasant. And, I think that the vast majority of participants here made much more than $50k/year (in 2014 household) dollars during the course of their career. So I have to disagree with most posters here and say that the article was actually right on. Sure, it helps (a lot) to be frugal, but it also helps to be lucky.