This isn’t true. You can install Linux on this, providing you disable Secure Boot. You can’t currently access the SSD, but that’s more the result of a driver not existing than it being inherently disallowed.
That's not clear yet. There is a NVMe driver available in Linux which works fine with pre-T2 Macs. On T2 Macs however the whole platform resets a few seconds after initializing the NVMe controller. The question is: Is that a bug in the driver or NVMe implementation of the T2 chip or something Apple does intentionally?
If intentional, this behaviour is nonetheless not documented in the whitepaper.
In such a scenario, a possible solution could be to offer an option to force an internal disk erasure upon toggling secure boot, in which case the internal device would be cleared for non-secure OS access.