The "rigging" possibility is just a maybe on top of that.
They ended up being almost literally bang on, ironically enough.
I'm seeing on Twitter that the app interface wasn't very intuitive and officials there were confused on how to use it.
Well other than there is no evidence of that and would that have even helped? My understanding of how the Iowa caucuses work ... what you describe would not advantage anyone. As I understand it the campaigns themselves are not reporting "I got X votes" directly.
I've not been to Iowa but I've been to some caucuses.. what you describe would not advantage anyone in my experience.
Buttigieg's comms director tweeted worksheets with the pin number to login to the app on Twitter https://twitter.com/bhalle87/status/1224558925946793985?s=20
If you don't know how the app works, how are you familiar with its security model?