You might want to look up the word king in an english dictionary. Oxford, Cambridge, and Webster, all define king as: "a male monarch" when it mentioned in conjunction, with a royal ruler of a territory.
Using the term king for a woman is not a grammatically correct, and is has 99% chance of being a mistranslation, from a gender-neutral term used in Japanese.
It's really not quite that cut-and-dry, particularly because deliberately using words 'wrongly' sometimes carries significant meaning. Using traditionally-male titles for female rulers has for instance a long, if unusual, history in actual politics (particularly, to imply that said ruler is not and will not be second-place, as female-equivalent titles have at many points been considered in many cultures). When those titles are used as such, using the male title in English is 100% accurate.
If the original text was a technically-potentially-gender-neutral word like 王 in a place where, for instance, a specifically female title 女王 would for some reason normally be expected, I'd consider it a translator's job to try to divine the intent and context and and pick between "king", "ruler", and "queen". Other characters
are referred to as 'queen' in places in the translation; if the translator does believe there's a salient difference in gendered language in the original that's supposed to come through, a translator has to figure out how to represent that. And indeed if the translator thinks it's important, "king" might easily become a potentially-appropriate way to highlight any unusual language.
(It could also be a bad reflexive dictionary translation as you say, or any number of other things, such as a translator's personal opinions on gendered titles vis-a-vis feminism, butch lesbian vibes for the hell of it, etc.—However, whatever the translator's actual intent, there is a fundamental difference between something being "factually incorrect according to the dictionary" and "a bad translation", because how we play with words is often
how ideas are communicated in fiction.)