Rsync depends on OpenSSH, but it definitely isn’t SFTP. I’ve tried using it against an SFTPGo instance, and lost some files because it runs its own binary, bypassing SFTPGo’s permission checks. Instead, I’ve opted for rclone with the SFTP backend, which does everything rsync do and is very well compliant.

In fact, while the main developer published a fix for this bug, he also expressed intention to drop support for the command entirely. I think I’m just commenting to give a heads up for any passerby.