I usually have a breeze setting up Jekyll on macOS, but this time I ran into problems that a friend of mine also ran into earlier this week, so it may be worth checking.
If setting up Jekyll is the first thing you do (often the case for me) the guide works great, but if you have added things to ~/.zscrc before, the chruby additions may fail.
This time, I had added case-insensitivity before activating Jekyll, which did end with this
autoload -Uz compinit && compinit
The reason why chruby failed to activate was because the first addition looked like this:
autoload -Uz compinit && compinitsource /opt/homebrew/opt/chruby/share/chruby/chruby.sh
Moving the source to a new line solved the problem.
If people adding things to the file without ending on a new line is common, then perhaps the Chruby setup code should be adjusted to start with a newline?