There seems to be no direct link between DDG and Yandex besides these two links that lack any evidence. Yandex merely appears to be one of DDG's content sources.
1. Yandex search results very closely match DDG's in Ukrainian (or did) as opposed to Google/Bing
2. When Yandex could no longer operate in Ukraine, DDG's region for Ukraine disappeared
I am reminded of Hanlon's Razor in this situation, that perhaps in Ukraine they simply did not fix their search after their top index provider was blocked.