FSD capability "noticeably departing" from EAP may be a wide variety of things initially. For example, as mentioned in my post above, FSD-enabled Teslas might be able to stop at stop signs or at red lights, or may be available for use on different roadways (surface streets) than standard EAP vehicles. And nothing in the tweet suggests that the driver will not need to continue to maintain control of the vehicle.
In other words, the tweet says there should be differences between FSD and EAP vehicles in 3-6 months, not that full self-driving will be available. Also, Elon said on October 19 they are hoping for a cross-country LA to NYC demonstration by the end of 2017.
Every new Tesla now comes with hardware for full self-driving capabilities It makes no sense to expect that customers will have full FSD functionality before the demo.