So HIPAA isn't rocket science and HHS provides plenty of HIPAA guidance. Kate's App isn't providing healthcare so HIPAA doesn't apply.
The landing page doesn't make it clear whether providers are expected to use it or not.
https://www.hhs.gov/hipaa/for-professionals/covered-entities...
HIPAA very much applies to this type of app or any other type of app that may deal in personally identifying information (PII) related to healthcare.
Edit: If no healthcare provider has access then maybe it could skate by. I interpreted "any user making notes to your account" to mean healthcare providers would have access. Even if not, they should still seek legal counsel. And this app is literally promising safety and security of healthcare information.