Paralax as i understand it is caused by the fact that any sight is offset from the line of the barrel so they can only intersect at one specific point. I doubt whether any of the focus ring scopes really correct for paralax unless you spend huge amounts of money (?). I've noticed that no body seems to spec the barrel to scope centre distance the scopes are corrected for. As usual I would expect this to be firearm biased. I have noticed that BSA list 2 air rifle scopes but haven't seen them in any shop. Back to red dots where I suspect a simular situation exists.

The red dot sight I am on about does truely project an image of the reticule onto the target. It is a true reflex sight. The company also did a telescope sight and that does work. This focuses the image of 2 circles and a cross to infinity. It does this by using a simple lense. The mirror and the piece of glass simpley reflect the image back into the eye. Do a web search on reflex sights telescope and you will see how they are made (and how to make one). Following on from this the same lense could be made to focus the image to any distance but I do not know if the lack of the need to align the eye would still hold. Some optical principles only work at infinity but sometimes this doesn't matter. A reticule that appeared to be behind the target may even be easier to use. The image would in any case be a hell of a lot sharper. Neither of the above sights seem to use this principle. I think that both are similar to the Cobra.
I have a Cobra sight. The red dot is clearly not on the target. (True of all the sights I have looked at so don't knock it.) This definately does show a tendancy to reduce eye alignment issues at a distance but I wouldn't disagree with the link provided above. (Thanks for that).