Miranda was supposedly a small settlement that went bad due to a "terraforming event" near the start of the Unification War. Jayne's comment about it not being a settlement (in the film) seems to hark back to an earlier version of the script. The line about it having 'settlements' was removed and replaced with a couple of different lines about a small number of 'settlers', but they evidently kept his line untouched.
"O brave new world, that has such people in it." It's called Blackrock now because it's not livable. Before, though, when it was new, when they were talking about flying in the first settlements, they'd named it "Miranda."
Serenity: Original Screenplay
Jayne is noting that this isn't a small settlement, it's in fact a major colony with multiple advanced cities and, potentially, millions of dead people, something that should be very much in the awareness of folks like him.