I have recently learned about magnetic induction and that it may not be earth generating our field as much as the suns magnetism linking to earth.

You can take a strong magnet then hold a piece of ferrous metal a bit away from it and it will act like a magnet itself just by being in the field.

So maybe Mars lost more than a hot core.

*fringe theory Mars liquid core was used to construct the moon which we have now to convey a whole civilization to a new planet.