If dry ice is flammable, why is it used for de-gassing of flammable vapours from storage tanks?

Wherever you heard that dry ice is flammable, that source is incredibly unreliable. Dry ice is made of carbon dioxide (CO2) which is used in fire extinguishers because it is not flammable. Dry ice produces vapor (CO2 gas) which prevents oxygen from completing the fire triangle, therefore preventing fire from ever happening.