Radiation hardness is different than sensitivity to EMP. It is the eddy currents from an EMPT that build up in and burn out small traces in micro electronics.
I'll offer a wild guess - perhaps they are only temporarily affected (i.e. mem-wiped) and function as normal after device reset, being made of iron, in contrast to semiconductor doping materials being hard-killed by the emp.
Drum memory [1] and hard disks [2] have been used in nuclear tipped missiles, which are supposed to operate in an environment where EMPs are expected. Both types use magnetism for storing data.