This is a really interesting idea that, as someone who is just a regular person who has no idea how planetary sciences work, makes sense. The implications are crazy for what this could also mean about life forming elsewhere. As far as we know, magnetic fields are vital for life as they protect it from harmful radiation. They do not have to be extremely strong either. For example, the Earth's magnetic field is weaker than a magnet on a fridge, so it would not take a lot to protect life if this "cold start" idea is correct.

The "warming-driven dynamo" presented in the study is the opposite of traditional dynamo-origin ideas, which propose that they form early in large bodies like Earth and then gradually cool.
In this model, molten metal blobs sink into Ganymede's innards to feed its core and whip up its magnetic field. They are warmed through two main mechanisms: radioactive heating and tidal heating.

First, as heavier radioactive isotopes decay into lighter elements, they release heat. Second, Jupiter's gigantic gravitational influence squeezes and stretches Ganymede as the moon whirls nearer and farther from its parent, as if "kneading" a planet-size piece of rocky, icy dough. The resulting inner friction generates heat. This heat powers the dynamo that gives Ganymede its magnetic field.

Altogether, this hypothesis does not preclude the possibility that Ganymede formed with a magnetic-field-producing core.