Since Ridley Scott filmed it as R, or unrated, he also has a lot more bargaining room when it comes to cutting it. He can just make smaller cuts to the more overt stuff until the MPAA and Fox are finally like "ok we're cool with that," while still leaving in there enough violent content to make it work.
While if he had filmed it as PG-13 off the bat, he'd have no room to maneuver. And we wouldn't be getting the unrated DC that we're likely going to see.
MPAA also doesn't care much about what's perceived as terror content. They tend to mention it, like with The Ring and "disturbing images," but they don't slap on an R automatically for that stuff. So we're still gonna get plenty of creepy visuals and an ominous atmosphere.
