That is why pretty much every western reactor has a reservoir of sand under the containment vehicle, if the worst of the worst happens, it melts into the sand which becomes glass.
Having seen the movie, suspect that a lot of people saw what they wanted to see, and not what the movie actually said. I'm fairly certain the comment about the core melting "all the way to China" was a joke/hyperbole.
Fusion OTOH is fusing together atoms of fuel that are relatively benign on their own - fusion is only possible at the high temperatures inside the reactor. I'm no expert on the physics, but AFAIK as soon as a fusion reactor is breached the conditions needed to sustain the reaction will quickly disappear and the reaction will almost immediately stop.
Even a fusion reactor has small thermal mass relative to the rest of the earth.