The wallpaper posted from Twitter is very close to the one demoed but it certainly is not the same. Perhaps an alpha version of the wall?
The sharp line jutting down the right hand side of the Twitter post is not present in the WWDC wall. Also, the dimensions are incorrect. As most know, a standard Apple wall in 2560x1600.
Adjusting the Twitter wall to a width of 2560 only proportionally yields a height of 1441.
Therefore, the Twitter wall will not be positioned correctly on the display. The WWDC wall's nebula is more centered.
This "blown up" affect may also account for the "more spacey" look of the Twitter wall as the stars appear larger in size.