I bet that's correct but do you think there could be some "cross platform" stuff in the works? Maybe a feature that apple doesn't want to announce prior to unveiling the 5S?
Such features are typically unique to the hardware of the new device. And since each device has its own software all they have to do is not put iPhone 5s versions out and that special feature is safe.
That said, frankly I don't see anything big and new. Not if this is truly a 5S. Fingerprint security sensors etc are more likely a redesign item. This go around would be more of a spec bump as all S models generally are. So perhaps more ram, better processor, better camera etc. Plus all the new API in the software updating things.
So there could be more Betas, particularly if there are still issues with core features and there are. After all, even if they announce the new hardware on the 10-12th, it could actually launch a month later if that's what Apple wants. And the software with it