I suspect that these are vastly different methodologies, as “heat-related” mortality is probably overwhelmingly existing conditions that are being exacerbated,

I suspect that these are vastly different methodologies, as “heat-related” mortality is probably overwhelmingly existing conditions that are being exacerbated, whereas the 18 here are probably direct “heat-related” causes, e.g. suffering from heat stroke. But they probably aren’t counting the person who happened to have a heart attack (which may have happened because of additional stress on their system, or additional dehydration, etc.) which would be counted in the first case.