I believe they are referring to the fact that linux (and non boot camp windows) cannot access the SSD on T2 equiped macbooks. People seem to disagree if it's the T2 itself or just a driver issue with apples proprietary controller.
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.
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.
People are responding this way because there are contradictory reports out there. Some sources, like the one I linked to and Apple's T2 security document, say you can run Linux without mentioning that you need an external drive. Have you tried disabling security as Apple suggests and installing Linux?
There seem to be several individuals making the claim that you can boot linux if you disable secure boot I have heard zero people claim that linux can access the internal device.
As far as I can see all primary sources are saying the same things. Then people who don't have the hardware are misreading said reports and spreading misinformation.
I don't have the hardware either so I can give you no direct report myself. I just bothered to read what people are saying instead of skimming and guessing.
