This week I saw the second instance I’ve seen of someone claiming that the Ancillary series by Ann Leckie is not science fiction. I’ve already ranted on this blog about how ridiculous it is to dismiss these novels for their choice of pronouns, but this is an even sillier tactic. It takes place in space! Where people talk to aliens and travel through gateways! The main character is a sentient ship! But nope, not sci-fi, nothing to see here.
In the most recent issue of Uncanny Magazine, Annalee Flower Horne and Natalie Luhrs make the case that these are all techniques long ago outlined in Joanna Russ’s work on How to Suppress Women’s Writing. The article is great (like all of this issue!!) and really worth a read.
Libromancy 
I haven’t read the criticism of the Ann Leckie series you are talking about, so I can’t judge whether or not that author has a point. Indeed, I haven’t (yet) read Ancillary Sword, so I can’t give my opinion as to whether it is or is not science fiction. I can say that without a doubt, women have written science fiction and I can give examples.
I will say that it is wrong to defend the claim that a book is science fiction on the grounds that it takes place in space or has starships or futuristic technology. Those are common trappings of science fiction but have nothing to do with something actually being science fiction. To make that claim indicates a fundamental lack of familiarity with science fiction that undermines whatever good point you wish to be making.
A good example is ‘Star Wars’. Star Wars is frequently misunderstood to be and misidentified as ‘science fiction’. In fact, it’s not. Star Wars is fantasy. This is most evidently seen by stepping back from the superficial trappings and looking at what the story is actually about. ‘Star Wars’ begins with ‘A long long time ago, in a Galaxy far far away’, which is a deliberate call out to the way a fairy tale begins. The story involves a young farm boy from a broken home finding a wizard, and going on a quest to rescue a princess from an evil black knight. The wizard mentor dies, but not without passing his power on to the young farmboy, who then uses that power to destroy a terrible weapon/monster used by the forces of evil.
This is not a story that is actually about science, or which explores science, or which really even cares about science. In fact, it has no science really, and has only the slightest of (largely unexplored) science fiction themes. The technology might as well be and often is magic. What is explored in great depth is the concepts of ‘good’ and ‘evil’ and the meaning of ‘heroism’.
One some good markers as to whether your story is science fiction or simply a fantasy set in space is whether it involves magic in some form (psionics, telepathy, esoteric mental powers, the force), whether it has a ‘chosen one’, and some sort of mythic story arc.
Conversely, science fiction is marked by actually caring about the science involved and being about the science. A very common core question is, “What does it mean to be human?”
My guess, based on just what I’ve heard, is that the Leckie book is actually science fiction. However, there is very good reason to believe that science fiction is increasingly a dead genre with fewer and fewer writers, and that increasingly what we see is other genres of story that are merely ‘set in space’ or ‘set in the future’ and wear the trappings of science fiction without actually being science fiction. That there is some crossover is not new; the boundaries between science fiction and fantasy and other sorts of fiction have always been blurry. That science fiction would no longer make up a large component of the bookshelves devoted to it is new.