Even with secure boot disabled you can't install Linux on the internal SSD. Installing Linux on a Mac has already been very flaky for the last few years, but now is impossible.
https://unix.stackexchange.com/questions/463422/how-can-you-...
https://unix.stackexchange.com/questions/463422/how-can-you-...
I wonder if Apple have an official stance on that.. i.e. "we're working on it", or "never".
Hopefully someone is working on the necessary driver support - these laptops are still very new so maybe nobody has gotten around to it yet.