It's really interesting, but the sample size is tiny. Only about 25 infants in each category, less then half of whom wound up hospitalized -- how could you possibly control for variables decently in such a small group? It would be neat to see if the results could be replicated with a larger study, though; I can't think of any reason off the top of my head to say it *couldn't* be true that breastfeeding does more for baby girls than for baby boys (with respect to severity of respiratory illness only).
And it's not scientific, but in my opinion there is something to the assertion that breastfeeding doesn't do much to reduce the *number* of illnesses a child gets. God knows my kidlet has been sick often enough. It's the *severity* that makes the big difference. All it takes is for your child to get the stomach flu once, refuse all solids and non-breastmilk liquids for a week and a half, and come out of it never even needing to see the doctor, to know how much breastfeeding matters.