I'd imagine Elon did not want to announce this on September 29, 2015 when the Autopilot 1.0 was not in production (after one year since it's introduction).
I don't think that's the reason. In my opinion, Tesla put a plate over the founders' extra camera(s) in order to keep this secret since people would have held off ordering a Model S, and some would have even cancelled orders, if they saw that the Model X came with more cameras than the Model S since that means the Model S's autopilot hardware upgrade is imminent.
Elon said that the Model S and X will have the same AP capabilities so it's safe to assume that when the Model X rolls of the line with more than one front facing camera, so will the Model S (meaning same capabilities). People have turned that statement around to mean the opposite, being that the Model X will have the same one forward facing camera as the Model S but that doesn't make sense to me. I think some people just wanted to interpret it that way but what he was really saying is that the vehicles will keep up with each other when it comes to autopilot, which only makes sense, at least in my opinion.
Again, only time will tell but at least it's soon now to see if my theory is correct since deliveries begin early 2016 and I doubt we'll see the same plate on production vehicles, especially when they don't show it in the picture. I bet that's when the founders' vehicles go into service to have their plates removed. I can't imagine the Model S will be coming off the line with one camera when Elon said the Model S and X will have the same AP capabilities so my prediction is early 2016 for new auto pilot 2.0 hardware on the Model S.
That's just a guess, though. If I'm wrong, it won't be the first time. But I think my reasoning makes some sense.