Correct me if I'm wrong here, but from a safety perspective shouldn't this mechanism fail open? Especially given the fact that I don't believe they even have a physical key, it's only the electromechanical locking mechanism
Nah, because otherwise it would be pretty easy to find ways to short out the battery, blow a fuse and open the car. This can happen on any vehicle which uses electronic locks and isn't really unique to Tesla. The difference is that Tesla has no physical backup key (I'm not sure if anyone else had dropped the physical backup), but it does have an external jump point you can use to provide 12v.
There are physical overrides on the inside though.