Ah, so it's just that you can play singleplayer already before release, but it will also be available to everyone who buys the game after release?
I interpreted this sentence:
VGC’s Modern Warfare 3 review says of the campaign: “Sold as a pre-order bonus and [...]"
...to mean that it would only be available to those who pre-ordered. Which would certainly be wild, but I thought, maybe that is why it's so short then.