Haven't read this in a while can someone tell does the uke tell seme that he was that girl...

turns out the seme already knew that the girl in the past was actually a boy

It has been heavily implied when you concentrate on Yoonseung and his mannerisms.
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There is even a chap where Yooseung asks Hakyung if he'd like to go to US with him bc SK is "cold". Hakyung thought it was absurd that Yoonseung wanted to permanently move to US just bc of the climate. But if you look at it from the cultural context, it sounds like Yoonseung is telling Hakyung they should move bc people in SK would not be welcoming of their relationship. Know that Hakyung was in front of him in a dress and wig as a woman when he said so.