Magnetocardiography is a thing. The NRO has some insane capabilities, so I wouldn’t immediately discredit the idea that a magnetocardiographic scan of a large area is possible.

It’s important to note that the individual was isolated by miles. And that they knew the time and location of the crash to determine the search radius.

It’s also one of the many tools they can use. So they may have used some combination of methods to reduce the search area and to pinpoint the target’s location. To say that they only used magnetocardiography is probably false.