They made the first prototypes/scripts for Bandersnatch in Twine, but they later moved to a Netflix-internal tool called "Branch Manager".
Cool, thank you both.
Inform 7 is interesting, but I never really liked coding in it; I feel like its almost-natural-English-but—not-quite syntax falls into a sort of “uncanny valley” of programming. Because of how much like natural language it is, it always feels like you should be able to write things in a lot more ways than you actually can.