Intentionally androgynous with those boobs? She's clearly just a typical tomboy character. A female using the masculine "I" (Boku) to emphasize being a tomboy. This is hardly anything new in anime culture. Also most of the dialogue in Japanese probably doesn't use pronouns at all given how the language works, so this is just the translator making wild guesses despite the main character explicitly pointing out that she was a girl 2 chapters back. If this was some sort of middle-sex character or something like that, believe me we would know because it would have made it into actual dialogue (JP authors love making these kinds of clarifications) rather than as just part of a fight's throwaway dialogue.
Last chapter the MC most likely said something along the lines of "attacks won't hit" (because its rare to specify details such as which attacks wont hit or who the attacks won't hit because its an action scene and details like this are already implied in the context), to which the MTL they probably used, guessed that it should use the pronoun "him". The MTL doesn't have any context for wtf the vague dialogue is talking about, and went for the usually safe bet of "him". This is normally fine as MTL screws this kind of stuff all the time, so its a known thing to look out for, but seemingly the scanlators instead took this at face value and assumed that the MTL had somehow deduced the character's gender being male.
This "they" nonsense in this chapter is probably a similar thing where the original sentence didn't have a pronoun at all and the scanlators just went with a safe approach instead of putting in actual effort to get the correct one. I see this all the time in novel fan "translations" (barely edited MTL pretending to be real translations) where characters who are explicitly mentioned as being girls are often mistakenly called "he" or "they" several chapters later, often times right after calling said character a "she" like a sentence prior, because the translator forgot who the characters were and was too lazy to bother checking if it all made sense. It's one of the easiest things to fix, yet somehow the thing that most often gets left wrong because translators seem to only care if the sentence if grammatically correct and forget that it also has to form a cohesive picture that fits the rest of the narrative.