Ok, I guess the first contact part ain’t getting through.
I understand what you are saying.
But I think the rule is more nuanced than that.
I have seen many instances over the years where a player gets the ball at yard line X, runs into traffic, the defense makes contact and even gets hold of the player, who then decides to run backwards to elude the defense and eventually find a lane to move forward. Or maybe to gain some time to get off a throw.
If this happens and the player gets tackled behind there the initial contact occurred, the ball inevitably is spotted where the player is ultimately tackled.
I think the player gives up the right for forward progress by continuing to try and elude the defense.
I don't know the rulebook well but I am pretty sure there is wording on this, because it happens all the time.
If Armstrong just gave up and stopped moving, I would have no problem with the ball being spotted at the 2 (or wherever the initial contact occurred).
The problem is that he didn't. He was too competitive for that. He is really good at getting away from defenders. Did it all game long. He tried to do it again here and failed.
That clear attempt to escape from the tackler and extend the play is what should have resulted in a safety call. IMHO.