As an embedded engineer, this is exactly why charging circuitry should be a closed system. For example, look at an Olympus battery charger for the camera. There is a chip inside that gives a little microcontroller information on the status of the battery. The micro can then tell the chip to reduce current to the battery or stop it completly. If an OS has access to this information and is actually doing the charging monitoring and the OS is hacked, well good by battery or even goodby other circuitry. The downside to this is that if it is a closed system, it becomes extremly hard to upgrade.
In my line of work, we run into similar situations. Not necessarily from hacking, just that if the software gets lost due to a bug, the low level charging software could get "stuck". What we have is a separate micro that we can update through a seperate PROM. The micro can tell the OS what the state is, but that is it. The OS has no control over the micro other than through a new PROM update. As far as a password, well, that sounds like the OS can get access to the circuitry. I'm not aware of how it is done in a mac book, but I know for our systems it is very, and I mean very difficult to update the PROM on the micro that controls circuitry. It cannot be done through the OS. If the OS tries to access the PROM, the OS would crash. A virus would have to re-write boot time binaries to be successful.
Lots of "mights"s and "if"s in that article, but if its a real threat to do more than just kill your battery, then I'm glad its uncovered. Having said that, there's no indication of any imminent threat. Fascinating stuff, though. I never realized the battery has a CPU.