Right-- believability is linked to this whimsical, pretty vague notion of the medieval/chivalric times they use as a template, as well as the in-universe worldbuilding.Basically yes, the modern person is so disconnected from what medieval times was really like that there is a sort of magical whimsy about that era in general. Which allows the world building to include magic and monsters without too much difficulty because the setting is already such a fantasy that monsters and magic don't feel out of place. The opposite of that is the sci-fi world either through Cyberpunk-like settings or full on space fantasy like Star Wars. Wizards, cyborgs, and technology that otherwise is basically magic still fits within the audience's mind because it's already such an unfamiliar setting that including that content isn't jarring.
Believability, immersion, realism, however you want to apply the description of a "world that makes sense" to the viewer is up to you. But that's essentially what I think the intent behind it was, in having a world based in such a way that the content within it and the lore behind it would make logical sense to the player. It's not attempting at being any sort of realistic interpretations of cultures in real life. In fact I'd say that FF16 goes through quite great lengths to sort of hide the cultures of the people contained within.
There are cities and towns and differences in building structure, but culture and belief systems are really left out of it, maybe to avoid pointless real-world comparisons or simply not wanting to take the extra effort in writing something most players don't give a fuck about. The story gives you just enough clues and tidbits to allow the player to sort of fill in the blanks on their own and allows them to make their own assumptions on the people and their ways of life should the player choose. But it's never outright laid out to you, even in the encyclopedia behind the scenes they keep it very generalized.
But if someone's notion of what they find 'believable' in a quasi-chivalric setting will include dragons and magic, but exclude people of colour, then that person is being utterly arbitrary. The former requires a far bigger leap.
There's a lot to unpack here. Firstly, the human tendency to recognise patterns. We all know about it, and how it explains why we see horses in cloud formations or Jesus in burnt toast. But you seem to just be using it as a catch-all explanation for a point of view you disagree with. You don't see X, so therefore anyone who sees X is just conjuring it from their mental programming. You haven't really considered the actual argument those people have made.You can never truly avoid some real world comparisons to be made, it's how people relate to a purely fictional story. So that is unavoidable to a degree, however that also doesn't mean you can always infer racist intent on magical people. The reason this happens is less about the artistic intent behind the setting and it's people and more a problem with the nature of the human mind.
The human mind has been programed to find patterns, and to connect dots with those patterns as part of our higher functioning intellectual capability. Just this past weekend Southern California got hit with a Hurricane (downgraded to a tropical storm by the time it reached inland) for the first time in 90 years. At the same time part of the Ventura basin got hit with an Earthquake for the first time in 90 years as well. So naturally the news and people where all over twitter (X) asking seismologists if there was a connection between Hurricane and Earthquake (no btw), because people try to make connections and make patterns fit and all that. Hell patterns are why Music Theory works so well, it's patterns.
So people tend to use that same line of thinking when looking at events in a fantasy game, or movie, which they then try to label as problematic because if you are already of that negative mindset of thinking everything is Anita Sarkeesian, then of course your brain is going to rationalize an explanation for why something is a social problem. Ignoring the coincidence of it, as writing tends to have tropes and creature types that fit story telling devices. This makes these kinds of jumps easy for people.
A good example is present in Harry Potter, where a group of people already where biasly determined to shit on the game because of their feelings towards JK Rowling. So what did they do, they looked all over the game (which they insisted they wouldn't buy or support) to find things they could use as "proof" JK and the devs are nothing but pieces of shit. So they accused the Goblins of being stand ins for Jewish people, and also found a horn used in Goblin raids that tangentally they linked to a horn some nazi used to raid Jewish village's....or something along those lines. Goblins have been based around the same monster stereotype for decades, if they have stereotypical traits based of a cartoonishly exaggerated real culture that doesn't make them racist or a problem. It just means that creators have built this monster around a certain inspiration that is then parodied and exaggerated to go from something real to something that's fake.
No matter how you slice it though, fictional creatures, settings, characters, all of that comes from some basis in reality, but in so doing it does not also mean a racist intent was behind or continues to their continued use throughout various works.
So in FF16, the 'pattern' is the lack of anyone who isn't white, in a huge cast. But that pattern isn't imagined-- its demonstrably true. Then we have possible explanations. In the case of patterns conjured by our mental tendency to invent patterns-- such as the horse in the cloud formation-- the alternative, likelier explanation is sheer coincidence. That's enough. But with FF16 we know it's not coincidence, because the publisher has said so: they've given us an alternative explanation. For that reason its not at all just a case of faulty pattern recognition. We have a definite, non-coincidental trend, resulting from human deciaions. And I can conclude that their alternative explanation doesn't hold water.
Now, you've also said that it's wrong to attribute these things to intentional malice. But that's not what (most) critics are doing. I don't believe at all that the creators were making an actively racist decision. But I do think they engaged in some pretty flimsy thought processes and gave a bunk explanation.