But there is no guarantee that any skin cells would be transferred via the sweat that does transfer from someone's arm to the inside of a jacket sleeve, especially if that person was wearing a long sleeve brown shirt.
Constant contact with sweat glands in neck, possible dandruff particles "glued" by dried sweat to jacket collar surface.Skin oil from fingers possibly embedded in areas in vicinity of bottom of zipper track when zipping....
If they can extract dna from a mummy they can make a match...... if they want to. But I doubt if they want to.
And if they did, you wouldn't suggest it was faked or planted? LOL.
You make a fair point about the neck and collar.