By the letter of the rule, yes it is FST... but where I struggle with it, is trying to understand why in this this situation it is a foul on the offense, when if he was on the line, it would be on the defense. This isn't a scenario where a back is 3-5 yards away from the ball, with no one directly threatening them. Arguably you could say that the back wasn't truly threatened, but the defender crossed into the NZ directly across from him, into his area of responsibility, and he reacted to that. I just don't see where the line is, that says this goes on O, or this goes on D, in this particular situation. What sets the two apart? Just because he is one yard off the ball? Wouldn't it make sense to extend the protection afforded the offense of being allowed to immediately react to a D player in the NZ in their area, to a player lined up one yard or less from the LOS, in their immediate area? Because if I'm a coach, why wouldn't I coach my DEs to try to get the back to move, since he is not afforded by current rule any leeway whatsoever in movement, and take my chances on him being in the NZ at the snap, because 50% of the bad things that can happen, are eliminated by rule (automatic DOF because lineman moved is out, leaving only being in NZ at snap.)
If the rule affords linemen protection from being penalized when a defensive player is in the NZ in their immediate area, it should also extend to the wing back on scoring kicks such as this (immediate reaction, in their immediate area, or directly in front.)
Help me understand they why part of this. Just my 0.02