Opening external editors from command-line is indeed not supported.
https://winscp.net/eng/docs/commandline#edit
Reasons behind that are discussed here:
Open file on server in external editor via WinSCP CLI
Anyway, I have added this request to the tracker:
Issue 2100 – Open external editor from WinSCP command-line
You can vote for it there.

Command line /edit now only support internal editor? How can I command line /edit with external editor?
I looked around and not able to find anything. Did I missed anything?