More questionable, P51guy, is how you could design a static port to freeze up at flight levels. Prior to the above, I don't believe there was any report of false altitude. A frozen static is more dangerous than frozen pitot.
The prior reports of events discussed a temperature rise, not a fall, as above. An iced TAT sensor will logically show temperature increase, as it becomes insulated from outside air, and the ice next to the sensor is warmed.
I am suspicious of both the DeltaNW event, and the NTSB release.
GB