Its the lines after silly. "But i have to be a boy", "its impossible to change", "but i cant cry" etc.. All the while Tamaki herself seems to have caught wind, and is subtly urging hey dog its ok if youre a girl. But lets go back to their first meeting where Kasuga is absolutely rocked that someone got to pass off as another gender, and wished Tamaki actually was a guy twice (!!) not because of something silly like basketball or romance (seeing as he expressed this long after their sports days and he knows romance is off the table as he is) but to know that such a thing was possible. But idk really weird to be in a rush to go "NOT TRANS NOT TRANS".
I didn't say Not Trans. I said that transitioning wouldn't be the answer.
And with how the entire thing is framed (his first statement in the chapter ending in "I gotta try and understand her"), I wouldn't say the situation is entirely removed from romantic feelings, though more specifically it's his feelings regarding Nekomiya Sr.
Both times he says "I wish you were a boy" it's directly related to how he sees their relationship: first time is framed as part of their dynamic in the sport, her perfectly following up his plays; second framed by how he cannot get a girlfriend but still remains so close with her - both times he wishes he could be fine with the distance, and suggests if she was a boy instead he would feel ok with it, but one is tinged by not knowing how to feel about her being a girl at that age, and the second with not being able to stop his romantic feelings for her when he clearly knows she'll never like him like him back.
And both times her answer, "I wouldn't mind even if you were a girl, ok?" it's trying to show how she appreciates him, basically saying "I like you for who you are", the first as assurance of their bond, the second as proof of how important he is to her even if they're not romantically involved.
There's parts that suggest he could be trans, the allusion to suggesting he wouldn't be able to come out certainly a sticking out point, not to mention the "If i was born a woman'...but again, the entire chapter does frame it mostly under the lens of how he feels about Nekoyama Sr. (romantic or not) than I feel you cannot make a fully honest point about his statements inequivocally meaning he'd be better of transitioning. If it was because he felt his body didn't fit him, he didn't feel comfortable being called a boy, or anything that does connect directly to him not being comfortable being a boy because of who he is, not because of someone else, transitioning might indeed be better for him.
If anything, I would connect that "boys don't cry" and other stuff that alludes to his gender (excluding "if I was born a woman/i wish you were a boy/I wouldn't mind even if you were a girl, ok?" that were discussed above) as issues with gender roles and expectations, with him having expectations of being a man that he cannot remove from himself, unlike Nekomiya Sr. who doesn't care about them (she is mentioned as doing "manly things"), which also are part of why he feels about her as he does: not as affection but as admiration and aspiration for who she is. Which still is a fine line between what might be subtext for being trans or for someone who doesn't wish to conform to gender norms, I'll admit (no, this isn't sarcasm or being cheeky, it's my actual opinion even if the way it's written CAN be read as if it was, it can be difficult to distinguish between both nowadays).
My point, again, isn't to argue on Trans or No Trans, as much as show that transitioning isn't necessarily ideal for him in this situation, with what we're shown as context for that kind of decision to be made, and that there's other stuff there too, mixed with the parts that can be interpreted as someone trans who hasn't fully gotten around to that identity. Basically, that there's more than just, or only, Trans subtext here.
