Or you just don't implement borrow checking -- which the language doesn't claim to have.
There are literally dozens of good enough small languages, e.g.: Nim, Zig, Crystal, Pony, etc etc, and at least the first two were written by a single person for the most part.
There are many other single-author single-serving languages (used to make games for example GOOL in Crash Bandicoot etc).
Not saying this can't be vaporware, but it's not beyond comprehension.
Heck, not long ago, a guy (with some psychological issues too) made his own OS, high level language compiler (HolyC), drivers, UI, and a flight simulator in assembler and HolyC, all working too (TempleOS).