That's not the same thing as being "an alternative UI for Yandex" - that's a partnership for one subset of the overall functionality.
More context https://kagifeedback.org/d/5445-reconsider-yandex-integratio...
Apparently they have been using yandex for 5 years as one of the search backends.
EDIT: this is what I found during a search. I did not claim they are fully backed by yandex.