you do actual measurements to see what factor the ambient temperature has on the measured temperature of a phone battery? How about having the phone touch a 37 degrees centigrade heat bath on one side (in other words: a human body) and ambient air on the other? Before this paper, could you have made any reliable prediction on how big an influence ambient temperatures were on the sensors in the battery?
Even under lab conditions, I doubt you'd get a good correlation between battery temperature and lab temperature if the phone were being used. Across an ensemble of phones and aggregating battery temperatures, you might expect to do better, but that would only prove this works in lab conditions.
These phones were sometimes indoors, sometimes outdoors, sometimes in bags, in conditions that are hard to replicate unless you have a very good model of average user behaviour.
We're working on ways to better detect the situation of the phone at the point of a reading, but even then it takes about 30minutes for battery temperature changes to take effect. If a phone was outside 15 minutes a go, the outdoors temperature will still have a large impact.
Modelling this or testing in lab conditions is not trivial!
There are mechanical engineering textbooks on the subject, such as http://books.google.ca/books?id=vvyIoXEywMoC&hl=en. Reliable heat transfer analysis has existed long before this paper was released.