Wasn't expecting the manga to actually explicitly confirms there isn't anything more going on or to be expected "behind the scenes".
It’s still “confirmed” through Hinako’s lens, when she herself is needing specific definitions. Sakurako’s “not dating” may be different than how Hinako interprets it.
Sakurako emphasizes things not changing and feeling secure, but we know canonically that she struggled with wanting to date, wanting more, and jealousy. So she’s either misrepresenting things, remembering differently, or has actually settled her mind on this in some comfortable way. Or, grimly, accepted that this is what she will get and puts a brave face on it. We don’t know.
Especially towards the end of the series, Yukiko seems to like ambiguity derived not from contradiction per se, but denying the reader access to the characters’ interior worlds omitting dialogue between Kasumi and Sakurako, instead showing them to us through other perspectives and interpretations from other characters.
Depending on the reader, the series is either frustrating bait or a walk through different types of love.
Personally I like Yukiko but this series still drives me totally up the wall, and that has much more to do with narrative choices than relationship ambiguity. I’m in the yelling and throwing things at the wall camp, and with a degree of projection without evidence that that is secretly how Sakurako feels too.