They Prey demo was another horrible PR move in a long list of AMD PR mistakes. It was supposed to be showing off Threadripper IO, instead it has caused people to panic and started the rumour that Vega is junk.
Yeah, that typo from Raja saying "about 60" instead of above didn't help either.
Two 580s in Crossfire get about 60FPS right now in Prey, if it needed two Vega Frontier Edition cards to do the same it would be a disaster.