So it's just that one specific slice of time and space that the Doctor can't get into, and that they happen to have been relocated in?

It seems a bit of a contrived hand-wave, given the amount of temporal chaos in the series as a whole (sending people back in time as a means of separating them forever doesn't make a lot of sense in the context of a protagonist who travels in time on a day-to-day basis), but presumably for some reason they didn't feel that 'Amy and Rory settle down, have children and live happily without the Doctor' would be acceptable to the fans otherwise :-p