NB: I DO NOT KNOW JAPANESE. This is all information given to me by someone who does read Japanese.
So the Japanese raws are not available yet, but there is a transcript of the text here. You can see at the end, is Hyuuga’s line:
The bold is the “onee” word that was mistranslated as the trans* slur. Remember that Reo’s teammates on Rakuzan call him “Reonee,” which as you can see, is a “word smash” of “Reo” and “onee.” Hyuuga just eliminates the “Reo” bit.
According to this Romaji dictionary ”onee” can, indeed, have homophobic and transphobic connotations, although its most common definition is “older sister.” Perhaps, it is similar to the English word “girl.” Depending on how “girl” is used and who it is said to, it can be made into a slur.
In short, if Hyuuga is transphobic for calling Reo “onee” then so is the rest of Rakuzan who call him “Reonee.” So, I’m sticking with Hyuuga being a word-vomiting doofus who was trying to be clever and failing (as usual). I say stuff I regret all the time, so he’s still my son & my spirit animal. ♥︎
We all know the Rakuzan team call Reo “Reonee” (Reo-sis) because they acknowledge Reo for who he(/she) is and know that Reo is proud of it.
Knowing Japanese (enough to hold conversation and translate stuff like books), the line 「オネエに興味はねーんだよ！他をあたれよ！」 could also be translated to “I have no interest in you! Go find someone else!” With オネエ (onee) being a Hyuuga calling Reo(nee) レオ(ネエ) by emitting the “Re” レ part ending up with “Onee” オネエ
So instead of saying レオネエ it became just オネエ
Transphobic connotations or not, it’s actually up to the people when said in Japanese since the way it was said is actually quite left open. More importantly, neither Reo or Hyuuga might even consider it as transphobic connotations since
- Reo is obviously flirting with Hyuuga since the begin of the match (and everyone knows it)
- Calling a guy like Reo as “sis” (onee) is actually quite often found when someone recognized their sexual preference, because of this, they refer to them as the gender they “wish they were born as”, especially when they are obvious about it
- The Rakuzan team was calling Reo “Reonee” throughout the match since it has started. (Hyuuga probably took the word smash and separated it)
The point is that there are lots of difference in usage of words in Japanese and in English. When a word may seems okay (or ambiguity), it could turn out downright wrong when used carelessly in English.