To energise the nitinol requires battery power at least. I would hope that it would be able to do that without being powered up. But yes, that detail is missing.
So, why wasn't a mechanical hook used instead? It seems to me they overengineered that part a bit.
Well, given that the Surfacebook utilizes computing power in the attached keyboard, maybe disconnecting it without properly checking processes could cause bad things to happen?
At least when it's on, it needs time to switch from the processors in the keyboard to those in the tablet.