Does Elastic DRS failback only copy the delta when restoring to the original server?

Yes, if you Failback using ISO to same original server then replication will sync only delta changes which happened on AWS Recovery server.

However, there is a feature in AWS DRS called "Non-disruptive failback testing" through which you can Failback to a different new server and in this case, entire data from AWS Recovery server will be sync'ed back to the new server.

(-) https://aws.amazon.com/disaster-recovery/when-to-choose-aws-drs/?cloud-endure-blogs.sort-by=item.additionalFields.createdDate&cloud-endure-blogs.sort-order=desc#:~:text=Non%2Ddisruptive%20failback%20testing

(-) https://docs.aws.amazon.com/drs/latest/userguide/failback-failover-notes.html