I think the app is not useful. I can keep track of readings in my head as the build goes along. For people who need an app to do anything it's probably OK. From a marketing perspective they have to have an app or customers will think they're in the Stone Age.
Referring back to OP it seems there's a basic point that has been missed. The tool accommodates double butted spokes and always has. Tool comes with a gauge to check spoke diameter and diameter of the butted section. Then there's a chart that converts the reading on the meter's gauge to a tension number or weight. It matters if your 14gauge butted spoke is 2.0/1.8 or 2.0/1.6 or 2.0/1.5.