It appears that they put an actual file system in front of S3 (AWS EFS basically) and then perform transparent syncing. The blog post discusses a lot of caveats (consistency, for example) or object namings (incosistencies are emitted as events to customers).

Having been a fan of S3 for such a long time, I'm really a fan of the design. It's a good compromise and kudos to whoever managed to push through the design.