Although Directory Opus has a setup dialog to register as the default FTP application, it doesn't seem to actually change anything on my PC. For example, it does not create a formal protocol association in the Windows Default Programs control panel applet like other software, such as WinSCP does. And if you try to create an FTP association, manually, Windows won't let you browse for Opus; you can only browse for Windows' Store ftp apps.
It works here. For example, typing this into a Command Prompt opens an Opus window:
start ftp://ftp.example.com
Yes, I guess I was referring to the aesthetics of program integration with Windows. Opus doesn't show up as the default app for the FTP protocol. No big deal. As you point out, Opus it is associated, but just doesn't give the user any indication it really is associated not under Windows "Default Apps" dialog