They say that the magnetic field detectors are intended to be used in conjunction with a detailed 3D map of Earth's magnetic field.
By identifying various kinks (anomalies) in the direction and magnitude of the magnetic field it is possible to identify exact points on the map.
Therefore there is no drift as long as you are not far away from the last identified point.
The only downside is that the magnetic maps need to be brought up-to-date from time to time, due to slow changes in Earth's magnetic field, and only few countries have the resources to keep such magnetic maps up-to-date.
Also .. the "coarse" map models (linked in comment) are good for five years, the enhanced models less so .. and there's those other factors.
It's also a wee bit trickier than you might think to (say) fly a route and identify your location only from DTM contour path directly under craft. (Subsitute mag vectors for DTM values)
It's more like old school missile plotting via DTM's where a preprogrammed "follow this" path is generated to facilitate optimal "when the value rises, turn here" instructions .. which are great "recipe navigation" commands .. until something goes wrong, it strays too far off course and has to recover.
Identifying any position via a "random" 50 line km slice is a much harder problem.
By identifying various kinks (anomalies) in the direction and magnitude of the magnetic field it is possible to identify exact points on the map.
Therefore there is no drift as long as you are not far away from the last identified point.
The only downside is that the magnetic maps need to be brought up-to-date from time to time, due to slow changes in Earth's magnetic field, and only few countries have the resources to keep such magnetic maps up-to-date.