We must have read different accounts. From what I've read, the guy left his unit on his own after making preparations to leave (giving his things away and writing a vague letter to his parents). I don't give any credence to the fact that nobody around him knew about his capability for desertion. There was no reported NK activity in his area. The brainwashing would have been only after the fact either way and really doesn't make a difference in the actual desertion itself. The only 'braing-washing' he could have been under before deserting would have been the NK propaganda about it being a socialist paradise. There should be further investigation at least, although the military has already done so and think he is a deserter.
A number of folks have looked at the "last" letter to his parents...and it doesn't relate or sound like previous letters which were in the parent's possession. Over the years, its been thought that the Army wrote the letter after he disappeared. I hate going that far in suggesting that some comander in South Korea might have been protecting his butt in writing such a letter...but stranger things have happened.