My god, you're right. I checked the raws and... even the raws are woke!
I can't believe how far this conspiracy goes... the woke sjw translators have even infected the original author...
(translator's note: 王様 means king, 女王 means queen. She's explicitly being called king here)
Actually 王様 is gender-neutral. 王様 "ou-sama" is the respectful way of saying 王 "ou", which if you look it up in a Japanese dictionary, you get this:
【王】の解説
- 国などを治める人。
(There are some other definitions listed as well but they aren't relevant.)
This translates to
A person who governs a country, etc.
Notice that it doesn't specify gender.
On the other hand, if you look up "king" in an English dictionary, you get
(the title of) a male ruler of a country, who holds this position because of his royal birth
In English, the word "king" is not gender-neutral. While in many cases translating 王 or 王様 as "king" is appropriate, this is not one of them. It's not like there aren't any gender-neutral words for it in English, either - we have "ruler" or "monarch" as two examples.
Of course, you might ask why the Japanese doesn't use 女王 or 女王様 "queen" here, given that it's the word that's usually used when they want to say "queen". It's quite clear why, as can be seen the line shown above 次の王様はアニス様なのか? "Is the next king [sic] going to be Anis-sama?" Here it wouldn't make sense to use 女王 "queen": the person currently on the throne is a man. Also the two people next in line for the throne are of different genders. Basically, what's important is the fact that they're talking about the one to rule the country, and the gender of who that will be is not known/not relevant. So the author has used the gender-neutral term, which in Japanese is just 王.
tl;dr: "king" is a mistranslation because it implies that they are using an explicitly male-gendered word to refer to the ruler of the country, when this is not the case in the original Japanese.