Testing for Thermal Issues Becomes More Difficult

Doesn't thinning make heat transfer easier at least to the package?

Yes, but it also makes thermal shifts faster, thinner packages have less heat capacity, and also crack easier due to more flexibility. The other thing noted in the article is thinner packages mean you have to use thinner metal layers which are what disproportionally do the heat transfer. Silicon transfers ok, but these modern nodes generate so much heat that silicon by itself is not sufficient.