The headphone can also be moving iron. The impedance can more easily, less difficultly be higher since the coil is not the moving part.
For example the Philips 4 transistor model KL5500:
http://www.extra.research.philips.com/hera/people/aarts/_Philips%20Bound%20Archive/PTechReview/PTechReview-19-1957_58-130.pdf
This seems to be the same principle as electric guitar pickups. The moving iron is the guitar string. Then a coil is would around a magnetic circuit from a permanent magnet.
See also moving iron or variable reluctance microphones:
https://sensorsandtransducers.wordpress.com/2012/02/11/the-moving-iron-variable-reluctance-microphone/
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