I think I found a use case Jolla had not considered. Or maybe they had but don't care. Or don't know how to fix it.

In short, the "use WiFi only" setting for dowloading updates is honoured only at the start of the download. A download already in progress is not paused or interrupted when you leave the WiFi area and the network switches to expensive mobile data.

[tl;dr]
I started downloading the update at work, about 30 minutes before going home. Risky but I thought, what the hell. By home time, only about 200 out of 400 MB had been downloaded. I had to leave sharp to collect the kids and cannot dilly-dally, so I closed the Settings app, assuming that the download would be interrupted and I would either resume or start all over again at home. So you can imagine my surprise when I checked it at upon arriving home and found that the download had finished and my credit reduced by amount roughly equivalent to downloading 200 MB through mobile data.