Just speculating here:
If the app is installed on your wi-fi only iPad, chances are it will work when you have a wi-fi connection; but not when you are out of range. So it may work at home, before you leave, but be next to useless if you try to use it on the road.
An app designed to work mobile, in a car, is compromised if it can't connect to the internet over cellular. That would be why the developer would list it as wi-fi + cellular compatible instead of just iPad compatible.
Normally the App Store would not let you install this app on a wi-fi only device, but perhaps you purchased it on a cellular enabled device first, and so had it available for download on the non-cellular device.
Or the developer originally had it listed as compatible, then changed their mind when people started complaining about it not working on the road.