hum... I think the irritation that some people are experiencing is that Girls Love as a tag tends to be viewed as girls love only since yuri fans are used to queer relationships being undermined by comphet stuff being stuffed in there, we can get a little understandably touchy about it.
At the same time, Bi erasure is definitely a thing and having some canon bi rep is nice for bi folk, really underserved market.
In addition, Poly relationships are also seldom represented in media, let alone represented well, so more poly content is good.
Do I think this series wove the bisexual throuple energy into the conclusion of this story smoothly? ehh... honestly no, but I think the main problem is the lack of nuance in manga tagging, there really isn't a tag on mangadex for poly relationships or bi content, both of which would have clarified things for those who go primarily by tags and would have caused people not here for those to tune out... and for greater numbers of people who DO want that to show up.
Blaming people for going more off tags and less on title is unproductive too, because titles can often be more poetic or vague, or lost in translation, or not fully representative, and people are conditioned by the tag system to expect it to provide greater clarity and hard data, because that's how website UI affects human psychology.