cordova or react native
ew
Well its not exactly a complicated app, I wouldn't be surprised if they use cordova considering it doesn't stick to any of the android design guidelines.
Cordova is a bit shark, or it was last time I looked at it, which was admittedly when it was called phone gap. But react native is a fine way of building a simple cross platform application which uses native widgets, it's just a different way of expressing your native views in a consistent way across platforms. You find yourself writing plenty of java and objective c or swift when using it. We're not exactly talking about the next grand theft auto here, we're talking about an app with half a dozen screens, a little bit of state, and a couple of calls to an http api.
Besides the point though, the point is it seems to have been released too soon.