My strong suspicion is that Ridley now intends to deal directly with both the alien's creation and LV-426/the derelict in AC and future movies, and that he'll use the same pat explanation for both: "David Did It." I think that's tacky and silly and far too overt, the idea of some evil android creating a monster and seeding the ship a handful of years before Alien. I think it's a bad mistake.
IMO all the explanation we ever needed for the alien or the derelict was in the original film and Prometheus. Ridley spent decades gabbing in print, commentary, interviews etc. saying flat-out that the alien was a bioweapon bred by the Engineers/jockeys that got out of control and contributed to wiping them out, much like it did the space jockey on LV-426. Prometheus backs all of this up onscreen by presenting another incident of widespread disaster that contributed to the same wipeout of the Engineer species; more bioweapons that were uncontrollable and helped kill them off. Everything Janek, etc. says in the film contributes to that same thesis Ridley has been pushing since the '70s. It was easy enough to connect the dot between what is uncovered in Prometheus back to events in Alien.
But when Prometheus got a mixed reception and Ridley was out of ideas for the sequel, he moved to directly prequelizing the original film and changing it up - now we have this new latter-day twist with David, and it sounds a lot like he is going to say that David made it all happen, not the Engineers/space jockeys. I think they're fixing what wasn't broken, and breaking it. I just hope they leave room to say David merely copied the creature the Engineers already created. It's all so unnecessary, but for hardcore nerds who love fan service it'll be a squeefest.