The article doesn't go into a lot of details, but it only says that the bug happens while in motion, not that the software update itself happened while in motion:

> The buggy update doesn't appear to brick the car immediately. Instead, the failure appears to occur while driving—a far more serious problem.

The problem is worse than "just don't update on the fly while driving". The update happened while not driving; the bug causing the failure mode of shutting down power and engine happened later while driving. There's nowhere to hide from these types of problems it seems.