I think they do all sorts of backup stuff - and as a consumer of S3 you need to evaluate how resilient their backing up is. S3 is pretty murky about permanence guarantees so I'd always look at making sure there is a separately maintained replication script to some medium I have control over if that data is irreplaceable and the costs of losing it are significant to the business.

Judging those costs and making that call is a complex matter of course.

What would such a service then do as an end-user beyond what `aws s3 sync s3://my/bucket /mnt/my/backup/drive/` does?

(Honest question, not provocation. To me either you trust AWS or any party to never lose your data [unwise], or you basically ask them to offer a way to rsync, which AWS does)

I think that's essentially what such a service would do. You might throw in some periodic less frequent syncs (maybe you sync down to the main backup every day and sync that backup to a secondary backup weekly or monthly) and maybe some of those successive syncs are done to hosts that are usually disconnected from the network to add in a firebreak.

They publish durability numbers for S3:

designed to provide 99.999999999% durability of objects over a given year