Was Kate Chopin truly a feminist?

Yes, both during her time and even by today's standards. You need look no further for proof of the answer than to her classic work, The Awakening. As you may know, The Awakening is about the life and decisions of Edna Pontellier, who basically rebels against societies expectations of what a woman and wife should be doing. She did this by forming a romantic liaison with a man named Robert. Did I mention Edna was married at the time? Ooh, scandalous. For its day, it was a juicy romance novel, as readers anticipated whether or not she would go with Robert or stay married. Edna was breaking out of what was deemed "proper" for a woman, and in doing so, Chopin, the writer, was challenging the mindsets of her day.
Interestingly, these same ideas are mirrored in Chopin's short story, "The Story of an Hour," when the narrator, Mrs. Mallard, hears news of her husband's death, and rather than experience sadness, she feels freedom.
What makes her a feminist:
1. Rebeling against societal conventions for a woman.
2. Voiced those opinions, even in light of skepticism and ridicule from peers.
3. She advocated for equal rights for women. (See "Kate Chopin: A Feminist, a Matriarch by Jenn Ropte).
Was Kate Chopin a feminist, absolutely!

The answer to this question hinges on how one defines feminism. The aims and goals of feminism have changed over time; however, by the standards of the late nineteenth century, Kate Chopin could be considered a feminist.
Chopin's stories are about how women are constrained by society's expectations. Women were expected to base their lives around the needs of a husband and children. It was assumed that any woman who did not marry was a sad individual indeed, an "old maid" who was to be pitied. Women who sought work or fulfillment outside of a homemaking role were seen as eccentrics at best and dangerously subversive at worst.
The Awakening portrays Chopin's most radical image of a feminist woman in her heroine Edna. Miserable in her marriage and unfulfilled as a mother, Edna is a woman who wants to be an artist. While Edna is presented as being selfish, Chopin still sympathizes with how she was essentially forced into her position by a rigid set of social rules.
We also see this in "The Story of an Hour," a darkly humorous piece about a woman who feels freer when told her husband has died, only to die herself of a heart attack when her plans are dashed after her husband comes home safe and sound. "Desiree's Baby" shows both the sexism and racism of the Victorian era, how a woman could be destroyed her the men in her life. She is totally dependent upon men, and had better hope to have men far kinder than the cold-hearted Armand as a mate.
These writings were challenging for their time. They openly question the way things are, the things people took for granted as true when it came to women, men, and marriage.
So, Chopin is pretty feminist. Her stories criticize how women are locked into roles that might not be for them (the character of Adele in The Awakening is an exception, someone who is fulfilled by motherhood, so Chopin is not critical of women who want this role or motherhood in general) and how women should be prized as individuals, not as subservient add-ons to their husbands.