No,
PiperCameron that is incorrect. The altitude in the ADS-B message is *always* in reference to 1013.25 hPa. And the reason is exactly as you state: A falsely set QNH on the altimeter would then report the wrong altitude. This way, ALL aircraft are reporting to the same pressure level.
Here a screenshot from RTCA DO-260B defining what altitude shall be reported:
