But it does not change the essence of the matter, since it does not work
But what does matter is my repeated question: “Does this specific file save correctly when you do that interactively in UI?”
And yes, it is the problem. If you open in LO the file you provided (specifying Win-1251), and try to save ad DBF with CP-866, you get this:
Error saving the document gek-1_2200_240221:
Write Error.
Cell B2 contains characters that are not representable in the selected target character set "Cyrillic (DOS/OS2-866/Russian)".
So yes, this conversion is impossible.