I usually use ST with, of course, syncthing running on both local and distant computers.
But is there a special way in which:
syncthing.exe runs on local computer
ST does not run on destination, SFTP is used instead (example: backup on a server for which I only have SFTP access but no possibility to install syncthing there)
There is no such mode, as a workaround you can run two instances of syncthing on one PC, one that uses the local data, one that stores its data in a folder mounted via SFTP. However this is quite inefficient (overhead for encryption etc. for a “local” copy), other tools like rsync are probably better in this case.