I always wonder: how many sensors are registering that temp?

Because CPUs can get much hotter in specific spots at specific pins no? Just because you're reading 100, doesn't mean there aren't spots that are way hotter.

My understanding is that modern Intel CPUs have a temp sensor per core + one at package level, but which one is being reported?

There's no way on Earth Intel hasn't thought of this. Probably the sensors are in or near the places that get the hottest, or they are aware of the delta and have put in the proper margin, or something like that.