Re: If Only....
For any unchanged block in the snapshot, the original filesystem is still the underlying source of the data
I guess it depends on how you understand the terminology. To me what you're describing is an incremental backup.
A snapshot to me is a backup of a moment in time but that backup contains everything needed to rebuild the system. E.g. dd if=/dev/sda of=/dev/sdc is a snapshot/backup, call it what you will, of sda that contains everything you need to rebuild that disc.