If it were something rubbing off from his thumb as opposed to something that were already there, the edges of coloration wouldn't be as finely contrasted. Also, it runs along the brim of his cap, but if it were to have rubbed off from his thumb, it would have been running from the edge of the hat toward his head, the natural direction the thumb points when you grab your hat. IMO, there's absolutely zero chance that it's from the rosin bag. I'll leave a one or two percent chance that the reason it's there is for reasons OTHER than doctoring the ball and cheating, but those pictures are pretty clear - it's a deliberately placed foreign substance.