In the paper (6.4 Limitations), there is an explanation for it:

..."Lastly, FeverPhone uses root-level access for reading a broad set of thermistors and a custom user-debug Android kernel for accessing raw touchscreen capacitance data. These requirements limit FeverPhone to phones with fine-grained sensor access. Temperature data from the battery thermistor is more readily accessible but did not yield comparable performance relative to temperature data from other thermistors. Still, future work could explore a more accessible version of FeverPhone with specific considerations paid to the battery thermistor. Similarly, the TouchEventAPI is publicly accessible at the cost of coarse-grained information about touch location. The use of this API in place of the raw capacitance data could be explored in future work."

At least in the apple ecosystem, I learned that any medical claim must be backed up by clinical studies.

My simple heart rate monitoring app was almost blocked for not having any studies, and I had to point to the dozens of other apps that simply report the heart rate from an external sensor.