The parent commenter is making that comment because this is precisely the nature of why the GPL license exists. Most of the processing of this application is FFMPEG, so why should someone who has done zero development on that library commercialize it?
Most of the processing of the application is FFMPEG yes, but there's a whole lot of application outside of the processing. Video editors UIs that don't make you want to tear your hair out are a valuable commodity and I think OP has the right to commercialize that if they want to. They just need to use FFMPEG in the right way as they do it.