Just common sense not to be waving your arms around in any way unless you are signaling for a fair catch. It's obvious some returners do so to fool the coverage and gain an advantage if they can get away with it.You are 100% correct and this seems to be a little known rule, but it's a rule so the returner can't "fool" the kicking team. Which in this case is exactly what he did - he waved everyone away from the ball, the Minny players were standing around the ball as it bounced then he grabbed it and took off running to take advantage of the element of surprise. It was the text book definition of why the illegal fair catch rule exists.
For DeJean to say he was trying to keep his balance by waving his left arm side to side is disingenuous to say the least.