Do you not see this act as being directed AT an opponent? Do you see it just as him celebrating what he knows is about to come, the TD?
I think there is doubt as to his intent and if it was intented toward the opponents, I just don't think a 1 second (if that) arms open motion to an oncoming opponent warrants that flag. High step, somersault into the end zone, hold the ball out toward them, flip them off, throat slash, change stride before entering the end zone, salute, bow at the waist, hand to ear, turning around and backing into the EZ, finger up to the mouth as if to say, "shhh" - these are all things that would qualify in my opinion. These are the kind of acts that are so clear to everyone that there is no discussion when they occur.
Could he have been gesturing to his teammates as if to say, "Hey - can you believe we pulled this off for a 50 yard score?!! This is great! " ? Maybe. There's no black and white standard to this one, so it kind of falls into that senator's description of pornography decades ago; to paraphrase, "I can't really describe it but I'll know it when I see it".
I think this just bought us a whole lot of unneeded grief down the road when slight acts such as this will go (rightfully, in my opinion) unpenalized.
But, hey - that's why we get the big bucks.