AltaVault only accepts SnapVault (type XDP) based Snapshot operations from ONTAP. SnapMirror is the overarching term applied to both SnapMirror and SnapVault operations for ONTAP, so I can understand why that is confusing. Please refer to the Data Fabric Solution for Cloud Backup Workflow guide for details about this.
The DR (SnapMirror) and backup (SnapVault) terms used in the above describe the aspects of the SnapMirror unified data replication architecture. AltaVault as a long term target represents the backup side of the SnapMirror stack, as opposed to hot replication (DR). Hope this clears up things.