My work is MIT, as is ts-fsrs. I said anki source code, but I was being hyperbolic. To understand the different exports of ts-fsrs I had to browse around in different repos at
https://github.com/open-spaced-repetition (particularly rs-fsrs; which I think is used in Anki proper) as well as read the algorithm
https://github.com/open-spaced-repetition/fsrs4anki/wiki/The... and infer by names being used.
I complain here, but this is open source and if I was slightly more ambitious I would submit PR with the missing docs (or at least an issue).
PS. my work is here https://github.com/runarberg/shodoku in case you are interested, but the source code bears the mark of being a weekend hobby project, so read at own risk ;)