The study is focused on the US, so it's hard to comment. In first-world countries, it's generally the most vulnerable who succumb to weather extremes, which means there are a lot of moving parts to account for when trying to parse statistics and identify a single cause.

Globally, though, it surely must be heat. It's simply a matter of population density in latitudes where heat waves occur.

All things being equal, it's harder to cool off than it is to warm up. (Though I confess I have had enough of winter already, and we have another two to three months ahead of us. Bah, humbug!)