Don't get too carried away, it's written for ksh and relies on bash and zsh to be compatible with ksh. You can see in the beginning it does a tiny bit of setup

Don't get too carried away, it's written for ksh and relies on bash and zsh to be compatible with ksh. You can see in the beginning it does a tiny bit of setup and never cares again which shell it runs on. For zsh, it literally says "emulate ksh" which is something zsh provides.