At that point, not only the rocket but also it's considerably more massive fuel are already traveling at speed. Any loss of pressure up to this point is partially offset by air entering the rocket from the launch tube. An open nozzle launch uses the original dimensions of the bottle's mouth for the nozzle. This is not the only way to launch these rockets, nor is it going to result in the highest performance. It is probably the easiest.
Some people like to fasten other sorts of nozzles inside the bottle mouth.
For something with as little mass and as much drag as a soda pop bottle, this can make a lot of difference. So far I have not equipped one of these with a cable-tie release, but it is in the plans. I anchor the base with weights, and don't have to fiddle with it again during a launch session. I will draw a picture of this as soon as I can get to it. A couple feet of PVC, two caps, two elbows, a tee and a reducer and the launch tube sleeve are all it takes.