Despite the naysayers, I expect the iPhone 5S to sell much more than 5M in the first week end. I wouldn't be surprised if Apple did aim to make 8-10 million 5S available in the first week-end, and launching in a lot of countries at the same time.
Look at what happened with the 4 -> 4S.
The iPhone 4 was a completely new case design and needed custom manufacturing techniques, and because of low yield rates they could only make enough to sell 1.7M units in the first week-end. The 4 availability was severely constrained during the first weeks (months?) as it was slowly released around the world.
The 4S though, came along when Apple had mastered the art of making a glass sandwich phone with a metal band around it, and was unveiled simultaneously in many more countries, so Apple managed to sell 4M in the first week-end, more than double the first w-e sales of the 4.
The iPhone 5, just like the 4, was a manufacturing nightmare for Apple because of the new casing and it took a while until they could ramp up the production. They still sold more than the 4S did in the 1st w-e, but could have sold much more if they had more to sell.
The combination of iOS 7, maybe some new hardware feature (fingerprint reader?) and a big worldwide launch could enable Apple to easily sell 10 million iPhone 5S in the first week-end. If they can make enough of them until this fall...
I think Samsung's comment is actually much more spiteful than it seems on the surface. In my view they're essentially saying: "Look Apple, we can take our time and still beat you to the market with our plastic GS4, while you struggle to find other partners to ramp up the production of the 5S and get those 10M units ready for launch day this fall".