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.
