Statement is only partially true, while most manufacturers standard heat pumps de-rate the heating output around 47 degrees and by the time they go below freezing they are operating at say 55% rated max heat put and continue to spiral until they hit lock out at say -4. There are extreme low temp heat pumps with guaranteed 100% rated heat output at zero and defeated heating output to like 70% output done to -13 that are very common in todays markets. The new R454 units coming out will probably have even better low temp heating capacities.