Which really goes back to the question of what constitutes a backup–see, e.g., Is snapshot and replication a ‘true’ backup?. And there doesn’t seem to be much agreement on that question–though saying that a backup must by definition be a file is just silly.
I think a more helpful way to look at the issue is to consider the threats/risks against which you’re trying to protect. Some possibilities:
- Accidental file deletion
- Drive failure
- Pool failure
- Catastrophic system failure
- Natural disaster
- Malicious data destruction
Snapshots on the same pool will protect against the first and possibly last (depending on whether the attacker’s able to destroy the snapshots). RAID will protect against the second (“RAID is not a backup” is, at best, incomplete). Snapshots replicated to a second pool in the same machine will cover the first, second, and third, and possibly sixth. Once you replicate them to another system–perhaps geographically-distant–you potentially address all of these risks.
While this doesn’t give a binary answer to “is X a backup?”, I think it’s a more helpful way of looking at the question.