This is cool but technically I'd say it would need to use the disk in the Mac to be running "on" the Mac.
Is the CPU/SoC not the thing on which then operating system is running?
If the tweet was claiming that it was ready to go for general use and that everyone should install it and use it as their daily driver, then I could see that the external boot disk caveat would be more significant... but it seems kind of irrelevant in the context of what they've achieved so far.