Is it really surprising that Bergdahl was unsure about what was happening to him? How would he know if he was being told the truth, or being traded to another Taliban group and just being emotionally manipulated? He may have ended up captured by his own foolish actions, but he is an American soldier held by a very hostile enemy. What dreamy ideas he had of the Afghan people prior to leaving his unit, he is now under no illusions as to their viciousness. When he climbed into the helicopter it was reported he wrote “SF?” and then broke down crying when he was told they were indeed US Special Forces. That response seems a more accurate judge of his feelings than his dazed and guarded appearance during the hand off.
Your interpretation presupposes that he was indeed held as a hostile capture by the Taliban. That is not at all a sure thing, considering there are reports of full on collaboration. James Rosen is working on an investigative report which appears to indicate collaboration of a very serious nature. This body language “analysis” would seem to be compatible with that situation, if he was working with them, felt himself to be one of them, of course he would be sad to leave them.
When I read that about his crying when learning that he was with the “SF” I felt that he was realizing that he was now in custody of the American military and his life was going to be on the line as a deserter. His gig as an honored guest of the Taliban was over!!