What isn't addressed in this 20-year plan is the risk of the many things outside of your control: health, local economy, unexpected family members, etc.
This isn't a risk-free option, it's an attempt to minimize risk in exchange for minimal financial independence. The margin for error is very low.
If you want to achieve financial independence in a more robust manner:
1. Save up 1-3 years of living expenses. This quickly gives you the financial independence of a practically unlimited runway. It's much more useful to have financial independence early in your career.
2. Use that independence to find the best possible way to make money for you -- retrain, start a business, etc.
3. Invest. Take as much (calculated) risk as possible while you're young and become more conservative over time.
People who front-load this in their careers often (in my experience) not only retire early but do so with significant wealth.