I would guess that many people would consider building an app like this without thinking too much about HIPAA. "We're not doctors, we're just building an app that will help people manage their anxiety." But the app clearly asks questions of its users that are focused on mental health. If a situation arises in which a user has a bad experience related to how their information was shared, it seems quite reasonable to consider whether their right to medical privacy was violated. This seems particularly important with the unfortunate stigma associated with mental health issues.
I don't ask this just to nitpick. I'm looking at building some projects related to education, and in education there's a comparable act called FERPA - Family Educational Rights and Privacy Act. It seems convenient to ignore these kind of regulations when building projects that are meant to be really helpful to everyone, but once a project like this takes off compliance with privacy acts seems critical.
I'm quite curious to hear from the developers what their take on HIPAA has been.