I'm not sure where you got the idea that I didn't disconnect the high-voltage, but I can assure you I followed the instructions and did not skip that important step. I intended to post something about my experience with the reconnection but didn't get around to it; this post reminded me about it.
I read/watched quite a bit on the act of replacing the 12V battery in the i3 and am supprised the bit of arcing/sparks on connection is not mentioned more. This is the only mention I see: https://www.reddit.com/r/BMWi3/comments ... onnecting/
It has been a few weeks, but I remember that there was some arching on make, but not break of the connection. After a second or two, there was a click like a relay closing and some other encouraging sounds of things coming to life, perhaps a fan or something. I watched the battery's terminal voltage on a multimeter when I made the connection and it quickly stabilized around 12.5V, basically the open-circuit voltage of the battery. A lower voltage would have indicated a significant current draw and I would have quickly removed the connection.
The arcing/sparks on the connection event are probably typical for people used to working with automobiles but may be supprising for techy-types. If you were to just plop the contact over the post, you may not notice the arcing, but if you are cautious about making the connection and touch the contact to the post watching for sparks, you will see them.