Pplkpr's emotion classification algorithm was trained on open data published by academic researchers. The Heart rate variability is based on the balance between the sympathetic and parasympathetic nervous system, which operate at different frequencies.
The GPS of the users are interconnected with the other users so that the app is allowed to know the users are hanging out with which person by their locations. But I doubt the precision of the analysis base on the locations because they do not include the occasions that take places in public spaces, for example, on subways, at a concert, or in a dining hall. In these places, people are close to each other physically, but they are not actually having a conversation or making any kind of contacts.