I asked Mary Jo about this schedule today and suffice to say she’s got great sources and stands by the information. So why the delay between RTM and GA (general availability, i.e. public availability)?
My theory is tied back to that information I received in July. I think that Microsoft is in fact going to continue updating Windows 8.1 past the RTM date—just as it did with Windows 8 a year ago, by the way—and artificially buy some time to fix bugs and clean things up. This is akin to a plane that pulls away from the terminal but then doesn’t head out to the runway until later, all the while claiming an on-time departure. That is, the August RTM date is essentially bogus.