Yes, but that only matters at full-throttle, when there's no way to cram more air into the cylinders. In normal driving conditions at part-throttle, the driver or the cruise control will open the throttle more to compensate for the thinner air, and airflow and cylinder pressures will end up being the same as if the engine was at low altitude.An engine is an air pump, and it acts like it has lower compression when you feed it thinner air.