That would be a layman's view based on the clumsy wording of the rule, without taking into account the how the rules are to be interpreted. It's not natural for a defender to defend with their arms behind their back, but they do it because if they naturally put their hands by their side, and the ball hits them, it's often a penalty. Defenders (like high jumpers) use their arms to help them jump, but that's also not an excuse for handling the ball.I don't think it was a penalty. If your arm isn't there you're not running or jostling properly, so its a natural position.
Jostling is not an excuse. If you're jostling and touch the ball, it's handball.