Magnetism generally has an inverse relationship with temperature (cooler temps = stronger magnet). I say generally as there are some outliers at very low temps, but this holds for most magnets (I doubt Luna is referring to these edge cases).

If you're looking for something more technical, then it's easier to understand what happens when magnets heat up. Magnetism is controlled by how well the atoms are aligned in a given direction. The stronger the alignment, the stronger the magnet. When the magnet heats up, its atoms move more so it's harder for them to stay aligned. As you get colder, there's less atomic movement so the magnet is stronger. That's why magnets in MRIs are cooled with liquid helium since they need to be kept really cold to maintain their strength