An idea
Maybe these represent single player...
And the video shown was multiplayer. Multiplayer is supposed to have 50+ players (not a big deal in of itself... see BF2, JO:TR, CS:S) but is also supposed to have destructable environments AND the ability to scan your own face/body onto your player.
Maybe those features, for multiplayer, required that the game scaled back in multiplayer?
I know I am grasping at straws (and by no means making excuses for PDZ) but... Rare is 1st party and has only been working on Conkers, Kameo, and PDZ forever (and Conkers is a remake!). They have a lot of access to MS tools and specs, long before 3rd parties...
You see the other top games that had video shown (PGR, GR, GoW, etc) and unless Rare has really lost it it just seems... unbelievable that it would turn out like that. Possible yes...
But MS had to know looking at the other titles and looking at PDZ that there was a chasm of quality. So maybe there is a reason for it??