Here is the thing: Temperature reading of battery may not even be the actual temperature value of the battery. Even the circuits itself that read temperature is sensitive to temperature (they have the same correlation with temp. as battery..Higher the temp. is, higher the leakage current, and hotter your device will be). So temperature reading itself will be combination of things, most of which have similar correlation against temperature of the environment. This is a well-known fact to all people working in semiconductor industry and we simulate things against temp. all the time.
And when you average out thousands of reading from different cell-phones, you will get rid of all those noise coming from phone-usage, whether person holds the phone or keep in in his pocket ..etc.
As a side node, if the batteries in each cell-phone made by different manufacturers, their temp. reading will also be different and will add a noise as well.
You built an app to read temp., OK fine. But, it just annoys me the fact that they talk about it as if this was a new discovery, and publish a scientific paper about it :)