Oxford study proves heat pumps triumph over fossil fuels in the cold::Published Monday in the scientific journal Joule, the research found that heat pumps are two to three times more efficient than their oil and gas counterparts, specifically in temperatures ranging from 10 C to -20 C.
So this is exactly what the article is about and up to -4 f heat pumps are more efficient.
If your the son of an hvac guy maybe your information is based on older installed units. I had a heat pump installed in my fathers home in the northeast ( non coastal) and I was shocked it ran well all year. I had heard the some rumor that you had.
Technology advanced and facts change.