How to interpret the raw temperature value coming from this sensor?
-20°C is your offset to the temperture at 0 LSB, which is 25°C.
So your reading actually means +5°C.
Notice how the specs in section 2.3 only guarantee an accuary of \$\pm 15°C\$. If you want to have a more accurate reading you will have to do some calibration yourself (read the value at a known temperature and correct for the offset in software).
The temperature sensors on these IMUs are there only to correct drift over temperature. This does not require them to be accurate, so they are not calibrated.
I assume adding them comes with basically no additional cost in manufacturing, so they just throw them in. But a calibration would be quite some additional cost.

I actually was wondering myself some time ago if the temperature accuracy is really that bad and wrote an email to the support team of ST. This was their answer:

The embedded temperature sensor can be used to perform an external
temperature compensation (when it’s not necessary to have a very
accurate absolute temperature value but only regarding the relative
accuracy). About your question, yes, you are right. In case you need
to measure environmental temperature more accurately, I can recommend
using one of our temperature sensors.