If you want to keep your zshrc "clean" you can add any number of individual scripts to `~/.oh-my-zsh/custom` and they will be executed for every new shell.
Oh that's good to know. For someone my age, I have embarrassingly little knowledge of shell customization. I just looked in .zshrc and there was a comment to the effect of "Add your aliases below, e.g. ALIAS blah=foo" and so I listened to it!
There is nothing inherently wrong with putting them into .zshrc!
The reasons I keep them separate is because:
a) I can simply backup the `custom` directory and restore onto a "vanilla" oh-my-zsh installation
b) I can use descriptive file names which would otherwise have to be comments
Also, some software likes to append stuff (like adding directories to PATH) to .zshrc upon installation so it gets kind of messy over time.
I guess the comment is from the plain zsh installation and oh-my-zsh just didn't remove it.