Their definition of a heat storm is 90th percentile temperature over an area half the size of Canada. I could not find the baseline 90th percentile mark, but it could be 1960 or 1980.

This is a made up definition. It mostly tracks overall extreme global warming since 1980. Warming oceans means automatic amplification of surface air temperatures over those oceans, and so under their definition, massive air heat storms are occurring just in every day conditions, under global warming.

A far more interesting study question is whether very strong long lasting high pressures are more common. Hurricanes (strong low pressures) have extremely amplified since 2016 in both numbers and strength events.