Linux is open. It's a lot more conducive in letting you know more about what's going on underneath.
To quote the original guide (http://www.catb.org/~esr/faqs/hacker-howto.html#skills2) "Under Mac OS X it's possible, but only part of the system is open source — you're likely to hit a lot of walls, and you have to be careful not to develop the bad habit of depending on Apple's proprietary code."
I've enjoyed learning hacking skills by configuring GNU/Linux partially guided by an excellent MIT course by MacOS users (freely available at https://missing.csail.mit.edu/). On MacOS you can get a lot out of customizing your shell, text editor, terminal multiplexer, etc. with dotfiles. But I prefer a more completely free (libre/open) operating system because it doesn't stop there, I've patched and compiled my own custom window manager as well, and my entire operating system has the level of hackability that I love in the terminal. I guess all of the software I've mentioned is not below the application layer but above other applications (except for the most part the text editor).
Is it for folks who are focused on going below the application layer (working on OS/Kernal etc) or just general. I find my MacOS to be such a comfortable system to hack and wonder if I actually need to move to Linux natively to get more involved in projects below the application layer.