Only if you put the 500 non-Montessori children in a separate school, with the same level of funding as the Montessori school. From the little that I know about these studies, at least some of the differences could be explained by the mixing of the 'control group' into an environment that is less ambitious or focused as the Montessori group.

(I walk pass 2 schools on my way to work each day - one Montessori, one not, sometimes around the time where children are dropped off by their parents. Even not knowing anything about the relative merits of the systems, I can easily tell which school I'd want to send my daughter to in a few years time; at the risk of sounding elitist, it's like standing next to the exit of an opera performance vs a soccer stadium).