Can anyone explain how he left a PFC and came back a Sgt?
He accrued the required amount of time-in-grade while being "held", and received an automatic promotion in absentia.
I'm not familiar with any of the more (last 20 or so years) recent conflicts/captives, but it was not uncommon for captives in the Viet Nam conflict to be promoted while being held captive. For example, McCain was advanced to Commander, and IIRC, Perot's running made, Admiral Stockdale was advanced to Captain while he was captive.