This is fine about the usage after installation.
He says he ran streamlink without ffmpeg.
FFmpeg comes together with streamlink-latest.exe, if you open installation file as an archive with 7zip for example,
you will find FFmpeg inside it.
After installation, I don't find any other ffmpeg apart from one I already have in c:\ap\ff\ffmpeg.exe, and on %PATH%.
My c:\ap\ff\ffmpeg.exe is one I extracted from streamlink-latest.exe before installing streamlink
and placed in to c:\ap\ff\ ( deleted zeranoe ffmpeg, replacing it with one found inside streamlink-latest.exe ).
I tried "streamlink" ffmpeg and "zeranoe" ffmpeg on their own ( no streamlink ) and found that in one
situation "streamlink" ffmpeg does the job and "zeranoe" ffmpeg comes half the way and stops.
https://github.com/streamlink/streamlink/issues/750
I try to find out what is he actually running, "streamlink" doesn't tell me anything,
and he is shy or lazy to say a few more words.