I get what you write about unsecure WIFI. But in this case that doesn't add up.
I use this app at home, with my own WiFi, since many years, and most of the time it works.
Just this moment it also works, with WiFi.
A couple of days ago, it didn't work, with the same WiFi.
If they wouldn't want that it works with WiFi, then they could make sure it is never used with WiFi, and they could inform the user at that time. But that is not the case.