Bridge Scripting: SFTP protocol not supported by FtpConnection

I'm trying to create a JSX script using the built in webaccesslib in Adobe Bridge CC 2018 that allows users to upload selected images by FTP.

I can connect to FTP OK with the webaccesslib FtpConnection, but when I try to connect via SFTP, I get a "protocol not supported" error despite there being examples of SFTP connections in Adobe's own documentation.

Has anyone managed to get SFTP connections to work using FtpConnection?