Actually no, Grindr is not a HIPAA covered entity (Such as a healthcare provider or clinic) or a "business associate" of a covered entity (Such as a Third-party to process and/or transmit medical data on behalf of a covered entity)
Now if Grindr had Grindr owned STD clinics that people got tested at and somehow that info got onto the app that would be a HIPAA violation.
An app asking you and you providing STD status freely, or any medical status for that matter, isn't. (Unless it's an app that bills itself as a medical something)