At what point does it become a FST? The rules and AR are pretty clear and they used defined terms with Lineman and Back. There isn't any gray area as to what a lineman is. Unless you want to "declare " that back to have been a lineman that's the only way. As the play showed he was pretty far off from meeting the definition of a lineman.
The player was a wingback, essentially one yard different than the lineman right next to him. Yes, he's clearly a back by our definition, I wouldn't argue otherwise, but his role and responsibility on this play is much more similar to a lineman than a back.
The defender jumped into the neutral zone so far that he was essentially shoulder to shoulder with the TE, within the gap that the wing was responsible for. The wing was "threatened" because if he did not react, the player would have been able to shoot the gap and been able to block the kick. The defenders action caused an instinctual flinch from the wing.
If we allow this to be a FST, I don't like setting the precedent that a defense can essentially "attack" a wingback without impunity and that wing has to remain still. If I'm a DC and in a tight spot (let's say Team A is 1/G at the B-1), I'm attacking that wing to see if I can get a free 5 yards.
Side note :
The definition that we typically use for "unabated" (2-18-2-e) states that it is DOF when the player "crosses the neutral zone and charges toward a Team A back". It is usually used when talking about the QB, but I could see someone arguing that it applies here.