The fact he brandished to his teammates and not to the opponent - to me - eliminates this as a threatening, taunting situation. I don't think it should have risen to the level of a 15-yard penalty in the game. Now, if he had stood over the opponent and brandished, then I think it should be an obvious penalty.
The very words the rules guy says is "if there is any threatening gesture, taunting, acts that provoke ill will or demeaning to the opponent or to the image of the game."
This was not anything done toward an opponent. So, the judgement must have been that it was damaging to 'the image of the game' (and maybe it was.) But, again, it is NOT automatic penalty, it is a judgement call.
To me, this seems like an opportunity for a referee (at this point in the game) to keep the flag in his pocket and walk over to #5 and say don't do that again.
Live and learn, though. Likely won't see our guy do that again.