@Mivas ...I've had to read the chapter about three times. the time they do choose to use a pronoun to specify Kou, the TL choose to use "they" and at other times they just used "Kou-kun" in context of the sentence (the latter more closely translated to Japanese sentence structure). Nao-chan is also refer to as "he" briefly in this chapter
and TL's choose to use s/he pronouns because if "they" was used for every sentence there is lack of context regardless of language. most other languages lack a proper translation for "I" so context of using ones name or nickname. but every language has feminine and masculine prefixes/suffixes to add. (it adds context to a sentence when you aren't specifying someone but also aren't naming an entire demographic) fan translated TL's are aware of the language they are translating the raw to (sincerely thankful because MTL is excruciatingly painful)
there isn't a need to specifically "know someones (gender)" unless you are intimate or want to actually know them. its the same as saying "my friend" which is gender neutral. intimacy as in you are some how connected to them, and want to know them for example is a business card or a social media profile
"Kou-kun" is probably refer to as "they" in preference that, Nao is now aware that their actually a girl. but also the fact that Nao who introduced Kou, his initial impression/relationship with them is that Kou was a guy.