Did you even read the paper? What the paper authors did was take a real heatwave (August '15 I think), model what the cities temps would have been with 30% more trees(something that requires a decent understanding of the effect you claim is just being 'discovered' here), calculate new death tolls from that, then compare the new death tolls with the real death tolls.

Your annoyance is misplaced, scientists know that trees make cities better in many ways, which is why they are performing studies like this one, to convince policymakers and developers to consider the benefits.