The whole point of the demo was to show off Threadripper and its I/O capabilites, not Vega. As such you would want to add enough GPUs to the point where there is no GPU bottleneck, and then show that Threadripper is still capable of keeping up. It's still a crappy demo mind you, as it should instead have been done as a comparison with a slower CPU, like they did with the DOTA2 streaming demo, which didn't feature FPS numbers either, but still did a perfect job of getting the message across.
Either way though this is all irrelevant. I just found
this 60FPS recording of the demo, and it quite clearly shows that the refresh rate was 30 Hz, not 60 Hz, so all of the number I mentioned above should be cut in half.
So unless crossfire scaling was terrible, a single Vega FE is basically the same speed as Fury X (possible 20-30% faster if they were running the 1.2 version of Prey). I can only assume that these drivers must be extremely premature, otherwise it just doesn't make any sense*.
*another possibility is of course that the frame with 2.7 tears, was very unrepresentative of the performance.