No, you read the datasheet wrong. It's +/- 3C.

I'm looking at the datasheet right now. Relative humidity accuracy tolerance is +-3.0, temperature is +-0.3. Different tables.

I imagine the temperature gradient across the car might be +/- 3degC, ignoring the actual sensor.

But the weird part is that OEM seem to be fine with that variation when it comes to climate control. Do they use multiple temp sensors?

Measuring temperature is not trivial. There's convective, conductive and radiative heat transfer and they all factor into the measurement. And that may not accurately reflect the "cabin temperature," particularly in a parked car.

It's undoubtedly more than that, depending on where you measure. The gradient between e.g. the roof lining and the AC vent could easily be 20C+ degrees on a hot day. Most boards on a vehicle will have their own temperature sensors to measure enclosure temps, and there will be zonal sensors at various points in the cabin as well. The climate control loop will be defined in terms of those zonal sensors.

I think the problem is not the accuracy of the sensors themselves but that its difficult to place them inside the car without influencing the measurement.