Nah its true. People never worried about this shit before they had multiple sensors on the die, but rest assured they've been playing with >20C deltas all the time. For decades. The cards didn't break either.
Sure, within reasonable limits, but 15-20C range is very narrow. I'd bump that up to 25C, so you can worry less and prevent voiding warranty or just damaging your GPU in the process of repasting. Because every time you remove that shroud, you are creating wear on the hardware, no matter what. Lots of people have worse or the same issues after a repaste. So they do it again... We know the horror stories on TPU: overtightening, too little paste, too much, uneven mount pressure etc.
ALSO, you're saying 90C core and 100C hotspot is a GPU 'in a better place, just toasty', but its really not. It means the GPU is probably throttling on core temp alone. I'll take the 70C core / 100C hotspot version, thanks.