That sounds like an awful lot of work to not send NTP requests over WiFi. Then again, I'm sure someone will inevitably find a way to make these things spray ice cream everywhere over WiFi if they do. Or worse, mess with the internal temperature or cleaning settings to make them not safe to consume.

It's actually probably best they never touch the public wifi. I don't know why no one has mentioned ethernet yet, though. These machines are static, they just sit there. There's already tons of wiring going everywhere, so I would assume that it should be relatively easy to get a cable drop to the ice cream machine. Then you don't have to worry about any of this wifi security. You could do port level security, but that's probably overkill for an edge network.

I have actually been wondering lately why there isn't more of a market for some of these things that it seems like you could hack together. There has to be something I'm missing. You'll never be a unicorn, because most of these kinds of sensors have a limited scale, but it seems like a good return on investment for a small team.