Sorry if James wanted to answer this….
I wondered the same and this was actually on of my proposed uses for the unit. (To log temperature in HVAC systems)
As it turns out the temperature sensor is internal to the chip and is mainly for temperature compensation of the readings.
As such it is not calibrated so can be a few degrees off of actual and it has a certain degree of self heating as the chip warms from sleep.
I think it is still a valuable tool as long as we are aware of the limitations and influences.