The phrasing is wrong. Since Infinite is a cross-gen title, they didn't show actual game footage because:

• Next gen is not official, so they cannot show something running on the prototype of console that "does not exist" yet. That would have forced them to release footage rendered on current gen and making all their effort "look bad".
• Showing footage of the game running on a next gen prototype or a PC might cause confusion later on. Things will certainly change a lot ultil the next gen console gets officially revealed, for good or bad, and that's risky (I'm looking at you, Wrecking Zone).

So, they went for the "engine demo" we ended up seeing instead of the actual, unfinished game 2+ years before its release.