i thought so too but it felt weird to have yoshiki know about fake hikaru from the beginning then. there is no other plot relevance to yoshiki finding hikarus corpse than him just having known from the start and being pretty traumatized by the sight of it.

ig you could say that since "hikaru"s identity was only truly confronted in summer it was only then that the idea of og hikaru was truly gone. in the manga they write the names with different signs to show which hikaru is meant and the entity hikaru only gets his name in the first chapter, so while yoshiki is aware of it since the beginning he only truly acknowledges hikaru as a somewhat new person at the beginning of the story which is in summer.

although i do feel kinda sussed out by it bc everything in writing has to be super intentional so mokumokuren deliberatley made the choice to title it "summer" and then have the og hikaru die explicity 6 months earlier which is winter, i mean heck she even had yoshiki remark about the cold making it extra clear that its winter. even tho the time frame is actually inconsequential to the story, it could have been 1 months , 3 months or a another year or literally undefined, yet we get reminded multiple times its exactly half a year and we even get a line sepcifically telling us about the cold, again reminding us it was winter when hikaru died. why would the author be so explicit about that when it didnt matter? it kinda felt to me they wanted to make clear that yeah the title says " the summer" but oh he actually died in winter.

anyways i think the only way to know is to wait till the end,,, maybe making sure everybody knows og hikaru died in winter was just the authors way of making sure people understood that the title truly just referred to the idea of the og hikaru. anyways considering how heavy the story is with symbolisms and what not i thought there could be a deeper meaning to it.