Basically, they need EBS without the single-instance (writable) mount limit, if they're using it to sync Drupal installations (php files, temp folders, etc.) across multiple web servers. S3 isn't really intended for that kind of workload (it would be possible to use something like s3fs, but I don't think that will give you a POSIX-compatible file system which is required for Drupal, and it's probably too slow as well (lots of small files, etc.))