i mean if youre considering only the biological part then yeah not incest but theyre DEFINITELY NOT like a childhood friends do lovers trope
childhood friends to lovers are NOT raised together in the SAME household since birth by the SAME parents being considered SIBLINGS and SEEING EACH OTHER AS SO (although yu is a weirdo and dont see it like that)
i see where your arguments are coming from but childhood friends to lovers and this are NOT the same.
You have considered everything except the blood relations which is the most important, it's still not incest. No matter how you perceive it, A and Yu has 0 blood relationship. You can be raised in the same house, be recognized as family, you will still have 0 blood relations. A friend can be recognized as family in a household mind you.

I knew people will be mad. I hope you guys dont take it incest since they literally have 0 blood connections. Take it like a "Childhood friends to lovers" trope. A and Yu's story is almost no different to Haebom's and Taesung's from Cherry blossom