That theory that the neck isn't actually lengthened is highly disputes since it wasn't made by anyone remotely connected to medical science or anyone allowed to examine the women since the Kayan don't allow it.
Keshishian, J.M. (1979), Anatomy of a Burmese Beauty Secret (155.6 ed.), Washington: National Geographic, pp. 798–801
Ironically you cite National Geographic:
Only problem is you've cited something older than you, probably. Because things change in 50 years sometimes.