Those snapshots are internally managed by the smbd service. Basically time machine backups are a sparsebundle volume (directory on SMB server) that internally manages snapshots, but those internal snapshots don’t do anything if the whole volume gets corrupted due to network issues, bugs in apple code, or accidental deletion by user. Depending on how share is configured, the smbd service takes a ZFS snapshot after each successful time machine backup, and auto-deletes its snapshots after a certain amount of time.