It uses barometer but uses a "baro-less" computer routine to approximate the barometric pressure based on the MAP sensor readings. By doing this the barometric pressure sensor could be eliminated for improved cost and reliability...nothing to go wrong with the sensor if it isn't on board anymore.
When the key is turned on the moment before the engine starts to crank the manifold absolute pressure is the same as the barometric pressure. When the engine is shut down there is no flow and no pressure drop across the throttle blades so the MAP is reading baro. The "baro-less" barometer algorithm grabs the MAP reading when the engine RPM is zero and uses that for the baro term in the calculations. When the engine is running and the throttle is opened far enough to eliminate and "throttling" the MAP will now be reading the barometric pressure again....theoretically. IF there is no throttling across the throttle blades then the MAP = Baro minus the slight drop across the air induction system. There is a second part to the baroless routine that allows the baro to update using the MAP value minus a preset pressure drop term that is looked up from a table of RPM. The logic allows the baro to update regularily as the car gains altitude as it is climbing so the chances of a deep enough throttle movement to update the baro is highly likely. There is also logic to always use the highest value for the baro term...whicher is highest the MAP sensor or the baro term previously calculated. This ensures that the baro term will also update descending as if the throttle is opened enough that the MAP is now greater than the stored baro term it will update to the MAP reading as the MAP cannot be higher than the baro, ever...unless the motor is supercharged..LOL.
There is a lot more logic in the baroless routine but that is basically how the car knows the barometer reading without a baro sensor. Most vehicles built today have a baro sensor again as the more stringent emission regulations do not allow the slight baro errors seen with the baro-less system.