Most people who rapidly lose weight will gain it back. The keyword here is most. There are exceptions to pretty much everything. There's no reason to doubt this person is one. That doesn't make their strategy good advice.
Before the regimen where I lost 80 lbs in 3 months, I regularly ate fast food, pizza, etc. After the regimen (off appetite suppressants, no longer calorie counting, no longer doing intermittent fasting), I still am doing keto, meaning I avoid the junk I used to eat that got me to current+80 lbs.
I suspect this permanent change in diet is what has allowed me to keep the weight off, though the more extreme regimen was what allowed me to lose so much so fast.
What about my story? I’m 170lbs down using keto and IF and have kept it off going into my 5th year. Also was an insulin dependent Type 2 diabetic who was off insulin 3 months in and haven’t taken it since.