> Their language, also called Kusunda, is unique: it is believed by linguists to be unrelated to any other language in the world. Scholars still aren't sure how it originated.
Languages with no known relatives are called isolates, and there are a lot of those: https://en.wikipedia.org/wiki/Language_isolate And languages do not normally "originate", unless they are constructed, like Esperanto, or arise in unusual circumstances, like spontaneous sign languages in isolated deaf communities.
> And it has a variety of unusual elements, including lacking any standard way of negating a sentence, words for "yes" or "no", or any words for direction.
As pointed out in other comments, not having words for "yes" or "no" is not very surprising. As for a "standard way" of negating a sentence, I wonder what that means. Kusunda has negative verbal suffixes, which vary based on some grammatical features, but so do many other languages. Location and direction is also usually specified by suffixes like in, e.g., Hungarian or Finnish.
See a grammatical overview for details: https://escholarship.org/uc/item/83v8d1wv