That reference is garbage. The temperature at which a liquid becomes a gas is the boiling point. At atmospheric pressure the boiling point of propane is -44F. According to the textbook Organic Chemistry by Joseph M. Hornback, propane has a boiling point of -44° F (-42° C) at atmospheric pressure. 20F is is a problem because it is getting closer to the boiling point at atmospheric pressure so that more external energy is required than can move from the exterior of the tank to the interior to prevent bulk cooling to the the point where the tank can’t make pressure greater than atmospheric pressure.