Of course they're aliens, just like the vast majority of aliens in TOS -- and most SFTV of the '50s through '80s -- looked exactly like humans.
As for the racial thing, I think it's widely misinterpreted. The episode says clearly that the Ligonians bear a "unique similarity" to medieval Chinese culture, so they weren't intended to be "tribal Africans," at least not by the scriptwriters. And keep in mind -- before this, the general trend in depicting humanoid aliens was to make them all white, or else cast white actors and paint them red or green or something. The one exception to that pattern that we saw on TOS were the Kohms in "The Omega Glory," and that's because they were actually meant to be parallel Chinese.
So "Code of Honor" was actually an attempt to be more racially inclusive -- to break free of that pattern of portraying all aliens as white. But, like many such first-blush efforts to break free of old prejudices, it didn't go far enough. Even though there was nothing specifically "African" about the Ligonians as scripted, the actors they cast used "African"-sounding accents (perhaps because most African-American actors would have more practice learning those accents for various roles than, say, Germanic or Chinese accents), and so it ended up conveying some unfortunate and unintended implications. As for the wardrobe and production design, it wasn't specifically African either -- the costumes were kind of a mix of Asian-influenced robes, Mideastern- or Indian-style turbans, and shimmery "spacey" fabric; but I guess the bare chests on the Ligonian men could've been interpreted as suggesting something "tribal" (though I think it was just Bill Theiss trying to apply his traditional skimpy design sensibilities in a more gender-egalitarian way than he had on TOS). So their intentions were good, but the execution left much to be desired, and some innocently intended ingredients had a regrettable synergy.
Part of the problem is that they went with a Katharyn Powers-cowritten script as the episode they chose to cast inclusively. Powers also wrote a number of early Stargate SG-1
episodes, and her writing of "alien" cultures (which in that case really were transplanted humans) wasn't much more nuanced or respectful or anthropologically coherent than this was. If the Ligonians came off as somewhat caricatured and barbaric, I think that was more due to Powers's limitations as a writer than due to any racial motivations. Or rather, if there were stereotypes motivating the script, they were more Orientalist stereotypes -- the old movie/TV trope of cultures from the Far East as exotic and sophisticated yet barbaric and prideful. Perhaps the original intent was to cast the Ligonians with Asian-American actors, so they were actually trying to avoid the obvious stereotype by casting a different ethnic group. Unfortunately, they chose one that's subject to plenty of unfair stereotypes of its own. So it was a misfire all around.