The author took the New Years dieting approach to FIRE. Cut out everything, make yourself miserable, and then quit.
Someone should have told him that the "FIRE by 30" crowd is only for high income earners. The math doesn't work if you're making $40k a year.
That doesn't mean it's worthless. For someone at his income level (assuming it stayed steady through his working career) it means the option of actually retiring versus working until you drop dead. And, maybe, if you start early enough and do find areas to cut, retiring early at 50 or 55.
But his biggest mistake is that his entire life has been built around spending money (friends all meet at pub, etc). When you cut all of that out, it doesn't leave you with anything else to do. It's why people who don't have hobbies fair poorly in early retirement. If your entire identity is built around your job, and then you quit it early, you're not left with anything.