Unfortunately I don’t currently own a cube i9, I’m waiting for the stylus version to be released later in the year.
Bonnom kindly posted an image of the boot order on an i9, which shows that the SD card is indeed bootable.
Miguel, please post your findings if you decide to try an SD boot yourself, (for linux). I’m totally keen to see what sort of performance can be achieved running a full install of ubuntu (or other distros) from SD.