What they are really doing is offloading the job of persisting data from ES.

So you wouldn't need to use S3 or some s3 emulator like other comments have suggested.

Give the ES cluster a disk it can write to that is guaranteed to be replicated (ie a RAID cluster) and thats it. The limits of object storage (no appending, no in place editing) tend to make those clusters cheaper so you could also run something like Ceph locally. It's probably going to do a better job of persisting data than ES also.