According to unverified information (but reputedly sourced from Boeing), a B777 comprises the following constituents.
Aluminum - 70%
Composites - 11%
Steel - 11%
Titanium - 7%
Misc - 1%
Boeing aircraft comparisons
I would consider that there is more than adequate amounts of metal in the wreckage of MH370 for a device that need not necessarily be a magnetometer - but is one that contains inductive proximity sensors, or uses induction balance or a pulse induction generator, as in most metal detectors - so that a satisfactory signal return could be generated upon proximity to metal wreckage that is buried in silt.
I am not familiar with the constraints or even availability of such products for sea-bed searches, but I just wonder if Fugro and the ATSB have precise knowledge of the constraints and limitations of the silt-depth ability of the sea-bed search equipment they are using, to be sure that they haven't missed wreckage that could be buried deeply in silt.
All the information I have gleaned suggests that sinking wreckage that still possesses considerable weight (and we're talking here of B777 wreckage sections that could still weigh maybe 50 tonnes or more), strikes the seabed with enough speed to ensure it could bury itself out of sight pretty easily, if it landed in the aforesaid deep, soft sediments.