Just a passing reference in the article that would seem, on the surface, to be utterly ridiculous. An entire planet's iron core was "somehow undone"? What exactly might that geophysical or chemical process might be? Did the strong force just up and decide to retire?
I think the standard view is that Mars’ core cooled down and largely solidified, thus cutting off its magnetic field (which could then no longer protect the planet’s atmosphere from the solar wind and cosmic rays). Hasn’t happened on Earth yet, because Earth is quite a bit larger and its core is still molten.
They’re probably trying to water this down to 3rd grader level and end up with stuff like that. The core is still iron or iron rich but between the overall mass and the lower amount of radioactive heavy elements accumulated further out it probably solidified and coupled up with the overall planetary rotation.