A lot of people make the assumption that having a large savings rate is too hard because you have to sacrifice enjoying life now. However, I never hear anyone counter that with the fact that those who are in debt to buy what they want now are paying the difference in interest. If most people lived debt free (waited until they could afford those things they want) they'd be well on their way to becoming FI just by never paying interest.
The second key to this whole thing is focus on what is really important to you, rather than desiring things thoughtlessly. Many who accomplish this thought process come to realize, "Hey, I don't really need as much as I thought. True happiness for me comes from X, Y, and Z." That pushes the savings rate even higher.
Suddenly, you're well on your way to being FI in an "extreme" manner. Once you don't have to work, the question to retire early presents itself. Some do, some don't, and that's ok. It's having the option to choose that matters. I think a lot of people watching us from the outside, just as this author did, look at FIRE backwards. Instead of designing your life in such a way that allows you to have more doors open to you, they assume we're depriving ourselves to dive headlong into the only door we can fathom anymore. For most people in this community, I'd say that's just not true.
The rest of the article is just quibbling. If you're worried about your money lasting through retirement, just don't thread the needle. Aim for a lower WR that will allow your investment returns to grow leaps and bounds beyond what your spending needs are. Maybe you then retire at 45 instead of 35. So what? You're still leagues above everyone else. Most people are in a much riskier situation by spending more than they earn and never confront the concern of, "What if I get laid off tomorrow?"