"access to markets" is not "the" determining factors, it is "a" determining factor. Have all the access you want via physical proximity, you also need to be able to offer something to that market, and an education is one of the factors in that. And it wasn't just missions that were the factor cited in the article, but a Jesuit one in particular: Jesuits are known for their strong push for education over the centuries. When the researcher here looked at non Jesuit missions in the same area, and therefore likely to have very similar "access to market", their outcomes were lower. I take your point about market access, and I think it is a good point, but education is one of the factors that gives a person access to a market.