I think I found the answer… walking through my thought process.
I didn’t recognize any obvious identifying features in the video. Doesn’t look like vim or emacs.
So, search for “tidalcycles editor” --> http://blog.tidalcycles.org/
As of now, this post is at the top: http://blog.tidalcycles.org/six-months-of-tidal/
Skimming a bit, I came across:
n "d7 a7 [c6 e] e7" # sound "rash"
This code won’t work even if you have followed the installation instructions perfectly. If you know even a little bit about Tidal, then you know that, for now at least, Tidal starts with nine connections to the SuperDirt synthesiser, named from d1 to d9, and this code does not look like even the most basic Tidal code examples, such as:
d1 $ sound "bd"
… It also took a few months to figure out that the talk featured an experimental editor for TidalCycles, which doesn’t require the use of d1, d2, etc.
Aha… the video you posted doesn’t show d1, d2 anywhere. So maybe this “experimental editor” is the one.
The readme for feedforward also mentions: “VU meters: To get in-text VU meters on patterns, switch on RMS sending from SuperDirt.” In the video, you can see blinking levels next to each musical element. So, more evidence in favor.
It was a bit of luck that this editor happened to be mentioned in the topmost blog post. Hope that helps.