I don't think hiragi like yuki romantically, it was more like yuki was the center of their little group and the one who introduced him to music and with him gone and mafuyu distant he just felt super lonely and ended up relying on shizu too much. Shizu loves him and would do anything for him but probably thinks hiragi is just confused and lonely. He didn't react at all to the confession, pretty sure he knew it was coming and didn't believe his feelings have depth. Sad thing is I'm not sure he would believe him even if they had sex. I think shizu has an inferiority complex towards yuki.
side story much much better than the original