http://www.buydisplay.com/default/7-inch-lcd-screen-tft-disp...
First one to do a teardown can tell us what panel it contains. I'm almost willing to bet it's that one, as the specs are close.
Still cheaper to source these parts yourself, you are right, but the official screen isn't meant to be the cheapest, it's meant to be cheap enough to be affordable, to workout-of-the-box, to be reliable, of good quality and have some guaranteed availability.
You can work with other screens and build and write your own interface if that's part of the pleasure you get from hacking on these devices, but for people who have other goals, being able to get an affordable screen that just works allows them to spend their time on other parts of their project.
It's good to have choice.