I agree with you on the first part, but are you sure that the heat from the fingernail is what's leaving that mark? I can take a cold object and run it on the receipt paper to get the same effect, so I think that's a different mechanism at play but I'm open to being proven wrong.

The developers in the paper only require a small flash of local heat to turn black, which is why thermal printers can print so fast given the time it takes to heat up and cool down the print head. Friction produces enough heat to do that. You can test this by pressing an object down only, or running it very slowly across the surface in comparison.