One of our foremost critical axioms is that characters should be human -- that is, even if they are talking animals or aliens, they should behave in a way akin to human psychology (or at least the pop-cultural understanding of it). Steven Universe, however, confronts us with heroes that at first appear entirely human and gradually reveals how their non-human physiology has created a distinctly non-human mindset. Which sounds very intellectual for a cartoon, but really it's an idea that is expressed more in humour than anything else.
This theme has been going on since "Together Breakfast" and "Cat Fingers", but really became prominent in "Giant Woman", when Pearl and Amethyst's discrete human bodies and identities proved not to be so discrete after all, and "So Many Birthdays", in which the Gems' immortality made them unable to relate to human social custom. "Steven the Sword-Fighter" riffs along the latter lines, creating a storyline in which the viewer is reminded of the Gems' fundamental nonhuman properties and how these can be alienating and disconcerting as well as useful and amusing.
The episode begins with a feint. Steven wants Pearl to teach him how to swordfight, and she obliges. This would appear to be an episode along the lines of "Cheeseburger Backpack" or "Serious Steven", in which the main conflict is Steven's desire to prove himself to the other Gems and the main take-away is his gradual growth into a hero. But everything changes when Pearl's training dummy runs her through with a sword, an act of sudden violence which, at least to me, was as momentarily shocking as the conclusion to Breaking Bad's "Half Measures".
When Pearl is stabbed, it's when she's at her most human: distracted by her irritation with Steven, her balletic sword technique is unable to parry the simple robotic patterns of the training dummy. She reverts to a gem, losing all qualities of a living being. There is a kind of tug-of-war to the episode's opening: first we have Pearl's humanity in wanting to help Steven, then an inhuman display of grace in the initial fight scene, and then her human irritation, and finally she is reduced to the most alien state imaginable. It would appear that, after a struggle, the Gems' alien nature has triumphed over their human appearance. After all, the fully dehumanized Pearl was able to best the semi-human one in combat.
The other Gems are nonplussed, calmly assuring Steven that Pearl will be back to normal in a few weeks. As in "So Many Birthdays", their physiology impacts their psychology: their inability to die makes the way they understand the world fundamentally alien. They can't even conceive of Steven's grief, and as such do nothing to try and assuage it. Steven, despite being a gem himself, is a viewer surrogate in that he's new to the life of being an immortal superhero and still has a fundamentally human and child-like psychology. Maturity may mean having to abandon that humanity in favour of the coolness of Garnet or the carelessness of Amethyst, both of which are powered by their essential invincibility. But for now, Steven is human and vulnerable and traumatized by seeing the closest thing he has left to a mother figure apparently killed in front of his eyes .
This could be very dark stuff, but of course it becomes a source of comedy in Steven Universe. Since the robotic Pearl triumphed over the more emotional one, Steven tries to take it on as a new maternal figure. He tries to teach holo-Pearl to act like the real one, but it lacks all of the real Pearl's emotional complexity, in particular the mix of affection and aggravation that defines Pearl's relationship with both Amethyst and Steven. Instead, it responds with the same simple aggression to everything. It is Pearl's equal in combat skills, and thus can match the supernatural feats that make her both a hero to the world and essentially inhuman. But it can't match the emotional relatability that makes her seem essentially human as well.
The episode ends with a physical fight between Steven and the training dummy, one that encapsulates the struggle between Steven's emotive understanding of Pearl and the dummy's physiological understanding of her -- that is, between the view of Pearl as basically human and Pearl as basically inhuman. Steven is able to defeat the physically superior dummy with his human ingenuity and goofiness. Shortly afterwards, Pearl returns in her full, relatable form. "Steven the Sword-fighter" sides, perhaps inevitably, with the more human and comprehensible part of the Gems' nature -- without letting us forget their less human attributes
This draws on a fairly conventional man-versus-machine narrative, in which humans' ingenuity and ability to feel emotions triumphs over the numerically superior machines. It's a narrative that's rather tired, and which does not recognize the ways in which humankind and its tools are so intricately connected as to be inseparable. If this story had been a two-hour movie, it would probably have been turgid and tedious, but a fifteen-minute cartoon series like Steven Universe can tap into our cultural knowledge of such narratives to create a brief and enjoyable story. It probably helps that instead of putting faith in the Enlightenment idea of the creative individual, as moth man-versus-machine narratives do, Steven Universe puts its faith in a maternal connection and human bonding.
You could read this as a reaction to the cartoon format itself. Like the Gems, cartoon characters appear human but are not, and their psychology can be both familiar and radically alien. In this reading, "Steven the Sword-fighter" is a plea to be patient with cartoon characters but also to be open to forming bonds and emotional attachments with them, recognizing their part-human nature (embodied by the voice actors and the writers who create the characters). There is, however, no need for this message: Steven Universe has already wormed its way into my heart.
This could also be reminiscent of the actual loss of Steven's actual mother, although the series hasn't revealed much about that yet.