I think you're greatly overestimating the problem. The only action a user can take in the app is to give a bolus. This is a relatively (1-2 years?) new feature that I believe necessitated FDA approval. Everything else is surfacing information (blood sugar levels and notifications) to the end user.

Edit to add: Utilizing a telephone is a significant quality of life upgrade from my (and many other) end-user experience. I'm guessing you are not Type 1 Diabetic (apologies if I am incorrect) in which case you have no idea the number of times in a day that one checks their blood sugar, boluses, etc. Frankly, it's exhausting. Being able to dial in a bolus or view my blood sugars from my phone, versus pulling out my insulin pump, has a significant impact and provides significant benefit.