"It was a strange feeling, and despite many, many long meetings with the editors, we couldn't quite pin down how this story would end. I often found myself thinking, "Is this story really interesting?" with a sense of unease. However, now I think it was for the best.
Just like in Volume 5 of the main story, Yoshida and Sayu's story has come to an end. In the side story of Mishima, her love story has also concluded. I wanted to give Yoshida and Gotou's love story a proper resolution. For that, prioritizing whether this was their conclusion was more important than whether it made for an interesting story. And strangely enough, by delving into this aspect, I feel like I found an interesting story.
Perhaps this will be the last time I write about Yoshida, but... even now, I still think he's an annoying, detestable person, and I can't relate to him. However, even so, after various encounters and through future encounters, I hope he continues to carve out his own path in life."
the aftermath makes it seem like he's still not really sure and just wrote it for the sake of writing it. I don't think most people would consider it canon given how OOC it makes most of the characters. and how he said "it was for the best" along with just like in volume 5 has come to an end makes it seem closer to an spinoff rather then an epilogue.
Don't think anyone should really take it too seriously if they don't want to. the books themselves are listed as sidestory/spinoffs to begin with.