Nobody ever said you can't disable secure boot and boot from an external drive. The point is that you can't access the expensive and essential internal storage where all your data lives. Here is an equivalent product a thunderbolt external nvme ssd 480GB for about $300.

https://www.amazon.com/Plugable-Thunderbolt-External-Compati...

If you don't mind spending hundreds of dollars, carrying around a second slightly awkward box wherein if you accidentally unplug it your computer crashes, and if you continue to use osx ferrying data between a and b periodically you too can run linux.

It would be utterly fantastic if people didn't keep responding to reports of the actual problem with articles like this which actually don't even touch on the item at hand.