How does snapshot work with sftp destination

I am trying to setup snapshot on a client’s site.

I have created an sftp account for them on my backup server.

I can connect to the sftp account with their credentials (using filezilla/command line).

I have created a destination in snapshot using the ftp credentials – the test connection button works ok including cd-ing to the destination directory.

Using sFTP connection

Connecting to host: xxxxxxx.xxxxx.xxx Port: 22 Timeout: 90

Login success.

Home/Root: /data2/home/xxxxx/

Changing Directory: xxxxxxxxxxxxxxxxx.com

Current Directory: /data2/home/xxxxx/xxxxxxxxxxxxxxxxx.com/

Success!

I configured a snapshot to backup the db and wp-content files to the configured sftp destination.

The backup has run and created the zip in the local uploads/snapshot directory

Nothing has been transferred to the sftp destination.

My question: Does the transfer of the snapshot zip to the sftp destination happen asynchronously? If so when?

OR, have I misunderstood something and configured the snapshot wrongly (though it does say under the destination column the name of the sftp destination)?