Kali Linux bootable USB not showing up in boot menu
If my memory serves me correctly, you can not boot iso9660 images from a partition on Intel Mac computers. At least not directly from the Mac firmware. I am not sure what the author of your linked reference was thinking.
I would recommend following the instructions publish by kali.org. Below are two webpages which together cover the creation of a portable Kali with persistence.
Making a Kali Bootable USB Drive (macOS/OS X)
Adding Persistence to a Kali Linux &quot;Live&quot; USB Drive
If you are still having problems, then update your question and post a comment below.