IIRC the point is that each NBD device is backed by a different S3 endpoint, probably in different zones/regions/whatever for resiliency.

Edit: Oops, "zpool create global-pool mirror /dev/nbd0 /dev/nbd1" is a better example for that. If it's not that, I'm not sure what that first example is doing.

In context of real AWS S3, I can see raid 0 being useful in this scenario, but in mirror that seems like too much duplication and cross-region replication like this going to introduce significant latency[citation needed]. AWS provides that for S3 already.

I can see it on not real S3 though.

Mirroring between s3 providers would seemingly give protection against your account being locked at one of them.

I expect this becomes most interesting with l2arc and cache (zil) devices to hold the working set and hide write latency. Maybe would require tuning or changes to allow 1m writes to use the cache device.