I think he's saying if you're in a bad urban school, it's hard to see how you're any less likely to be in the top 6% than if you're in a bad rural school given

I think he's saying if you're in a bad urban school, it's hard to see how you're any less likely to be in the top 6% than if you're in a bad rural school given the same academic abilities. (Academic abilities which don't always need to be very highly developed.)