Since the baby has to squeeze/press/suck the nipple against the roof of the mouth in order to get milk, I'd say is the former: teaches proper swallow. If it wasn't like that, John Mew wouldn't think it as ideal to breastfeed for years.

EDIT:

I found a letter from John Mew asking at what point does adult swallowing develops. I don't know why there is only one page, but it is interesting nonetheless. The author who answers also states, that nonutritive sucking habits among other things encourage atypical swallowing.

My understanding is that extended breastfeeding helps prevent the child from generalizing a suckle swallow pattern beyond weaning and the transition to solid foods. I guess the idea is that bottle feeding or introducing other food and drinks before the child is developmentally ready to adopt a mature swallow pattern encourages them to stick with the infantile technique.

For what it is worth... I nursed my four children for about 16 years combined and about 4 years on average. None of them had a problem nursing. They all weaned on their own and I let them just eat adult food with me (we've always ate organic Weston Price style food). All of them needed orthodontic work to various degrees mostly because they had mild tongue ties that were in the way of the right bite development. I'm actually on this forum because I'm determined to find a way for my 13 y.o. to develop his face/jaws. 

Conclusion: it's always better to breastfeed than not but sadly there are no guarantees. 

Thats very interesting, i wonder if its just because of the tongue ties