In this case, I'd view it more as an interactive mode to use ffmpeg?
That is, it isn't like emacs is the thing that is doing the trimming. It is just providing convenient access to ffmpeg to do that. That it is able to do this using roughly 300 lines of code is quite impressive and largely speaks to why people love doing things in emacs.
It would be one thing if this was code golfed to get to a working state. Reading the code, it is remarkably straight forward interactions with ffmpeg.
Emacs more like a platform like DOS or the Terminal/Shell/TUI combination than an editor like Vim/Micro. The wealth of libraries, all centered around the concept of buffers and windows, make it easy to integrate pretty much any other tools.
Is it an REPL? use comint.
Is it a tool that act on text files (lint, test,...)? Use compile
Do you need input with completion? Use the minibuffer.
Do you need multichoice selection? Use a buffer and mark stuff (there's a mode that you can extend from if you need to display tabulated data).
Do you need to define flags on the fly? Use transient.
Network request? Calling shell command? A combination of all the above? Emacs got your back.
And because it's a live programming environment, you can start quickly with a prototype (or just altering stuff) and then tweak things until you have a proper package.
That is, it isn't like emacs is the thing that is doing the trimming. It is just providing convenient access to ffmpeg to do that. That it is able to do this using roughly 300 lines of code is quite impressive and largely speaks to why people love doing things in emacs.
It would be one thing if this was code golfed to get to a working state. Reading the code, it is remarkably straight forward interactions with ffmpeg.