But my point was that if you want to backup such a volume (the encrypted LUKS volume), you will have to backup the whole volume, and can't do incremental backup

But my point was that if you want to backup such a volume (the encrypted LUKS volume), you will have to backup the whole volume, and can't do incremental backup. If you want to do incremental ZFS send/recieve backups, you have to do it on the unencrypted ZFS volume, and thus your backup wont be encrypted. That is you can't have both encrypted and incremental backups at the same time. With native ZFS encryption you do get that.