My guess is that he’s doing an extreme version of Lean FIRE. I know a few folks doing less extreme versions. They own little, don’t go out much, and save every dime to retire as early as possible.

At least one person I know who did it retired in his mid 30s, then came out of retirement when his investments crashed at the beginning of the 2020 lockdowns. His skills were out of date so he started making way less than he was making when he retired.