Well,
I think we're confusing the issue a bit. There is a significant difference between income during working years and income requirements during non-working years. Without a relatively high income (defined here as middle class not 'high income' according to the statistics) during your working years FI is difficult, but not impossible. As a previous poster said, the key here is what you do with the income during your working years and how you build a lifestyle that you enjoy.
MMM would probably not be retired now if he had only ever earned his current net spending. In fact, it might be said that we would have been, and likely still would be poor. However, he earned a much higher income, saved most of it, paid off all purchases and acquired his toys relatively early on and feels no need to replace them with more expensive toys. This is where the middle class goes off the rails with a 'spent don't save mentality'.
Poverty can, and probably should be thought of as income (or ability to provide) relative to needs (actual needs, not wants). Part of achieving FI is effectively 'pre-paying' some of your needs during your working years so that you don't have to pay for them during your non-working years, while at the same time working on reducing these needs to limit your dependence on consumption.
MMM and other FI folks have imputed additional spending of 12-20K per year on housing (paid for), 2-5K on vehicles (paid for and cheaply at that), and savings of 3-5K (assuming like most middle class folks you only save 5% per year). Add this to MMM's reported spending of ~28K per year and you end up with a squarely middle class lifestyle at 50-60K per year 'spent' after tax.
I suspect the definitions do reflect close to reality, but most peoples perceptions of needs are vastly inflated hence the lifestyle requirements to be 'middle class' are actually the issue.