Updating Software is different from updating OS.
That's why updating a BIOS is the kind of thing you do on a sunny day, on a battery back up.
One wrong move and its dead. You have to flash it back into existence.
The download+install are probably in one progress bar. If you turn it off during the download nothing should happen. Dude on twitter didn't say (and probably doesnt know) during what stage he unplugged it.
I get paranoid as hell about any of my consoles when they are in the middle of a long save or update. Making such a huge update a day one thing is bound to bite some impatient people in the ass though.