The very front tip of the ball only has to cross the FRONT of the line, not cross the entire line. And that's the ball in the circle, not his arm.
I understand the rule, but I think that you are mistaking his forearm for the ball. Compare the position and direction of his white armband in the 2 pictures. If the ball were a brick, I'd agree with you, but the curve of the ball appears to be enough to just avoid crossing the line.
But again, it is so stinking close that the margin of error should result in the touchdown, we're talking less than an inch. Just like with the margin of error of the spotting of the ball before measuring for the first down. At some point it becomes so close as to be impossible to tell the difference.