There is no truth whatsoever in that quote your posting.
Unless thermodynamics have changed since i went to college a proper heatsink can’t actually make the core temperatures go up. A mistake he might have made is sticking it with ordinary double sided tape, or worse, double sided foam.
And about insulating the CPU/GPU, that’s what the black packaging with the text on it is for 🙂
In short: a heatsink is always a good idea, but unless you’re overclocking your RPI1/2 or running it in a hot environment it’s not necessary. They do recommend a heatsink for the RPI3 though.