> Why would I use it for s3?

You have it the wrong way around. Here, ZFS uses many small S3 objects as the storage substrate, rather than physical disks. The value proposition is that this should be definitely cheaper and perhaps more durable than EBS.

See s3backer, a FUSE implementation of similar: https://github.com/archiecobbs/s3backer

See prior in kernel ZFS work by Delphix which AFAIK was closed by Delphix management: https://www.youtube.com/watch?v=opW9KhjOQ3Q

BTW this appears to be closed too!