Yup, we don't know much about the planet, it could be smaller or of lower mass compared to Earth, making for a cold core, like Mars. It could be closer to system's sun enabling human inhabitation. IMO, it's the mistake of the viewer to assume a molten core.
And if there were a molten core, being exposed to space wouldn't freeze it. Without some sort of medium to conduct heat into (such as atmosphere or water) it would take a very very long time for that much heat to radiate away into space.