The moon appears (from the article) to have a solid iron core, and a liquid layer above that.
Four things could be keeping the centre of the moon hot enough for all this action:
Gravity (Tidal) heating
Magnetic heating
Radioactive heating
Initial frictional heat of formation + retention.
I’d have to guess that radioactive heating and initial frictional heat are the main components, just like in the Earth.
Heavier (radioactive) elements naturally shift in towards the core of planets and planetoids during planetary formation. They produce heat, and that heat has nowhere to go. The ‘hot’ area of the Moon is tiny compared with the Earths, but apparently enough to maintain a ~ 80 mile liquid zone.
An 80 mile liquid zone is tiny compared to Earth’s. In addition, would a molten metal core need to be rotating in order to generate a magnetic field? Reason I ask is that Venus likely has a sizeable molten core, but no where near a magnetic field, and it’s rotation is much slower than Earth’s. I do however realize that correlation is not causation. The lack of rotation likely due to tidal lock.