I agree. Snapshots are absolutely a backup, albeit a backup with certain characteristics that make it primarily suitable for restores in-place and unsuitable for remote esp. off-site backup to non-ZFS boxes i.e. incapable of ZFS replication.
However ZFS Snapshots can be far a more efficient way of holding backups because the only disk space used is for blocks that have changed.
And ZFS replication to another server can be far more efficient than file backups because it uses snapshots to work out what block have changed and replicate only those blocks.
A backup does NOT have to be a file, however a backup to a file is definitely a backup, one with different characteristics to a snapshot.
I would imagine that you can probably use qemu-img (or even dd) to convert a Zvol to a VMDK as a means of backing up a VM Zvol to a file. And you can presumably pipe that into e.g. zip or similar to compress it still further if you wish.
(I suspect that qemu-img would be more intelligent about e.g. not backing up unused portions of the Zvol than dd.)