I think, you are mixing two different protocols - SFTP and FTPS.
Filezilla server (as far as I remember) is an FTP server with SSL/TLS support, which is called FTPS. This is same old FTP with encrypted transport.
SFTP, on the other hand, is actually SSH file transfer protocol and is not compatible with FTPS.
Duplicati supports SFTP. It may support FTPS with FTP backend, I did not check…
I can’t find quickly of Filezilla server started supporting SSH, but any other SSH server will work as SFTP backend (OpenSSH, Bitwise, etc…)
EDIT: Just checked and both ftp and aftp backends support FTPS with appropriate parameters.
EDIT 2: From Filezilla wiki: Support for SFTP (SSH File Transfer Protocol) is not implemented in Filezilla Server.
https://wiki.filezilla-project.org/FileZilla_FTP_Server