Interesting, I did not know the specifics. But that is one possible way to limit it vs. trying to say hitting 70 five days in a row in Feb is a heat wave. I believe the whole premise of the phrase use is to warn people in specific situations about danger. People who work outside, sports players, people (especially very young and elderly) in non air conditioned locales. So, agree that hitting 70 five days in a row in January is not a heat wave.
Maybe you can also say 90 is the min but it also has to be 7 AN so 90 in August does not work.