Lol is right. Take the situation to an extreme, put the cone say 50 or 100mm in front of the muzzle, and very few would argue that it has any stripping effect.
The other extreme is with the cone touching the muzzle, and there too there can be no stripping.
So the optimum must be somewhere between the two.
It's not difficult to show with a bit of maths that the stripping effect (if there is any) should start to appear when the tail of the pellet has a quarter of a diameter of clear space between it, call that 1.1mm to 1.2mm, and the muzzle.
When the air leaves the muzzle it is still at some considerable pressure relative to the atmosphere, and will be looking to "leak" in all directions, not just in the direction of pellet travel. The air has some momentum it is true, momentum being the product of mass and velocity, but the mass is very small. It is still trying to get past the pellet.
At this quarter diameter distance, the cylindrical area exposed by the pellet is equal to the area obscured by the pellet, and the air willl choose the path of least resistance.