Benning discussed the Pick 6 play on his radio show this morning. He said there were 3 things that caused the INT return for the TD, which was a comeback route on the sideline.
1. The WR made a bad break out of his cut, and didn't come back to the ball very aggressively/downhill. Coach Williams immediately ripped into him.
2. Armstrong didn't make a great throw, as his throw was to the inside, rather than the sideline.
3. Chris Jones made a great cut on the ball and great play.
So, Armstrong appears to have not made an accurate throw on a sideline comeback route, but that is a long throw, and we know that Armstrong's struggles with accuracy at times. However, the pick 6 wasn't entirely Armstrong's fault, and there also should be credit given to Chris Jones. If the WR fights to the ball more, the pass is probably just broken up and the offense moves on.