Wikipedia defines feminism as seeking "to define, establish, and achieve political, economic, personal, and social rights for women". It also calls The Left Hand of Darkness part of "feminist science fiction".
[spoilers for the book follow]
The science-fiction novel describes a culture in which there is no gender divide: individuals are essentially hermaphrodites and commonly both father and bear children during their lives. One of the major characters in the novel is an earth male who finds it difficult to understand the non-gendered nature of this society. They, in turn, find him bizarre and believe him to be some sort of pervert.
It is a book of considerable depth. The author uses the non-gendered nature of the alien culture to explore human obsessions with duality, and with gender roles in western culture. It poses a number of interesting questions for the reader to consider.
However, while the book has strong and clear themes around gender, I have always struggled to understand how it is explicitly feminist by the definition given at the start of the question. It seems to posit not particular superior judgement of this necessarily gender-equal society compared to our own. Nor does it seem to offer much exploration of patriarchy or other societal ways in which men oppress women in human society.
How, then, is it considered feminist?