It's not too different from having a C compiler written in C. Once you bootstrap[1] the first version, you can create subsequent versions from the first one.
[1] https://en.wikipedia.org/wiki/Bootstrapping_%28compilers%29
[1] https://en.wikipedia.org/wiki/Bootstrapping_%28compilers%29
However I apparently added some error in the language and the compiler was then broken, so I couldn't make my fix and re-transpile without checking out a clean version of the repo :D