The point NoSaint is making is that Mills was out there for both. It makes the reason he solely was difference between sacks allowed under center and sacks allowed out of the shotgun unlikely.
But to your point NoSaint, could it be as simple as the gun, while worse for TT's passing ability, better for his elusiveness? Has to be easier to avoid the rush when they take wider angles, less up front pressure, etc.
Furthermore, I think we need a contrast between 2016 and 2015. He was a much better producer in '15 and for all we know, he murdered it in the gun versus under center that entire year. Context and all that.