That is a hypothesis. It was put forth by a single scientist 100 years ago and has been accepted as a given without question ever since.
Look at some fact:
Mars lost its atmosphere, Mars has no mag field;
the moon has no atmosphere, the moon has no mag field;
Venus's atmosphere is not rotating, Venus has no mag field;
the sun has an extensive rotating atmosphere, the sun has a huge mag field;
Jupiter has a huge rotating atmosphere, Jupiter has a huge mag field;
earth has some atmosphere, rotating, earth has some mag field.
Mars has a rotating iron core but no mag field. The sun and Jupiter have no iron core, but guess what.
Therefore, the iron magnet hypothesis was only a guess, and there is no reason to accept it as reality. Look at the atmosphere. Look at the sun with its recent sunspots and mag storms. Don't look directly at the sun without proper eye protection--have to say that because there are litigous morons about.
This is the first I've heard of this. Apparently it doesn't have much of an active core, or there'd be plate tectonics. Is it just a solid core, or is there liquid churning as well?
Sorry for jumping in a little late but
Titan has no magnetic field but it has a thicker atmosphere then Earth's and it appears to be rotating quite fast.
While Mercury has a magnetic field but for all intensive purposes has no atmosphere
So I don't see how an Atmosphere can be causing a moon/planet to have a magnetic field