“Atomic decay inside of the earth heats its molten core to a temperature that is hotter than the surface of the sun!”

First, the molten outer core is not due to this decay. It got that way during formation of the Earth, and is kept that way by the heat of crytallization as it adds to the solid inner core. Since heat moves from high temperatures toward low ones, and the core temperatures exceed the mantle temperatures, then the flow of heat is from the core to the mantle and toward the surface, not the other way around.

Second, as the MIT report mentioned above says, the heat seen in crustal rocks derives both from “tectonic” sources and from the decay of radioactive rocks ***concentrated in the crust*** at around 10 km.

This is not to say that other portions of the Earth don’t have radioactive rock components, but only that the heat “signal” seen in crustal rocks is dominated by radioactive decay in deep crustal rocks, and by tectonic heat, which may include heat conducted from the core, but also by near-surface proximity to magma and the mantle.