An important thing to remember is that energy waves travelling through a medium aren't the same as the medium moving itself. So while the bang may be reflected back via the stripper there may well be significant amounts of air moving forward still. Stick some powder paint down the barrel behind the pellet and fire it at paper. See what happens.
When Jon did my stripper there was a magic distance between muzzle and stripper. Without wanting to give the secrets of his work away I wouldn't have thought the air would have much chance to get out but it would certainly remove the air travelling along side the pellet.
Perhaps someone will one day make a trap door that closes behind the pellet