It's difficult to put into words, but there are two ways to look at any given story: from a Watsonian point-of-view (that is, from within the story itself: discussing character motivation, plot development, etc) or Doylistically (from without the story, taking into account the context the writers are working in: why they made certain narrative choices, the existence of themes, symbolism, etc).
For instance, let's say you see two same-sex characters on television hold hands under a rainbow. From a Watsonian point of view, it's just two characters who (for whatever reason) decide to hold hands after the rain. The characters don't know what the future holds. But from a Doylistic point of view, audiences can safely assume that the writers are trying to convey something about their relationship by positioning them under the universal symbol of the gay rights movement. It would likely be foreshadowing for the fact they'll eventually fall in love.
In Finn's case, from a Watsonian POV, obviously his skin colour is largely irrelevant in the context of the galaxy in which he lives. As far as I know, there's no racism in the Republic. But from a Doylistic POV, the fact that a Black man is playing this character inevitably brings connotations to the story, whether the writers like it or not. As the article points out:
In The Force Awakens, Finn seizes his freedom from the First Order by escaping from the clutches of Kylo Ren with Poe Dameron. He discovers an identity by discarding the white stormtrooper helmet that erased his skin color and dropping his slave name, FN-2187, to become “Finn.” When Han Solo, Chewbacca, and Finn capture Captain Phasma — Finn’s former commanding officer and master — he taunts her with the phrase, “I’m in charge now,” which expresses his embodiment of Black resistance.
People could saying that the article's writer is reaching, or that this would have all played out the same way with a white actor in the role, but with John Boyega as Finn, that reading of the text exists whether JJ Abrams is aware of it or not.
It's clear that he didn't, and we're left with a trilogy that decided to jettison the possibility of Finn leading a Stormtrooper (that is, slave) uprising in favour of yet another story about why we should all feel sorry for violent white men who murder people. Along with the bait-and-switch of the first TFA posters, and Finn's gradual sidelining into less important subplots, there's stuff like the fact that Kylo beating Finn into a coma at the end of TFA is barely a factor in the following two films; instead the tiny little scratch on Kylo's face is given huge thematic weight, while the inevitable existence of a serious wound on Finn's back is completely ignored. And the fact that Disney was clearly more comfortable in Rey kissing a space fascist that spend most of his screen-time assaulting her over the Black guy who genuinely loved and respected her speaks volumes. The optics, Doylistically speaking, are Not Good.