When the anthology puts so much emphasis on there being girl x girl relationships and literally has yuri in its title, it’s only natural people would assume it be yuri-only. And I would like to ask the question, have yuri-only anthologies/mangas ever straight out said “this is yuri-only”??
Even with so much emphasis on yuri relationships and yuri in the title, I didn't once assume it to be yuri-only. It doesn't seem natural to do so at all as far as I'm concerned. To answer your question, I don't really know if anything is ever identified in that way, but I would hope that they wouldn't go the exclusionary route. I'd hope that most publishers of works by the LGBTQ+ community wouldn't use that sort of terminology even if the stories contained within were literally only yuri and nothing else. We want a community of inclusion, right? Besides, what benefit would people have knowing that there's only yuri and nothing else in it?
The issue seems to be the binary between 'homosexual' and 'heterosexual' content, wherein people assume that a relationship could only really be one of these- for instance, even if one of the people involved in a lesbian relationship was bisexual and had dated men before, her current relationship would be homosexual, or in this case, yuri. So if the story focuses upon this relationship, it would be yuri, and that's all the classification you'd need. Even if ex-boyfriends are mentioned or star as characters, they wouldn't be part of this relationship, and so there'd exist no need to further specify the yuri tag. However, the existence of polyamory and polycules fundamentally challenges the monogamous relationship model that society often considers a norm, making it possible for a relationship to be simultaneously homosexual and heterosexual- not either yuri/yaoi or het, but both at once, at the same time.
So the polycule in this case would be both yuri and het, which falls under the yuri implications of the title and marketing. However, many readers of yuri have a visceral reaction to seeing a guy involved in a lesbian relationship owing to all the 'yuribait' stories they've suffered through, which involve a shoehorned het relationship at the end of an ostensibly-lesbian romance, and so they react with anger. The reality is that this is something entirely new, something that covers largely uncharted ground, and so the existing tags and classifications of monogamy-normative society wouldn't suffice to describe it. And to consumers looking for reliable, standard narratives of yuri, the new formation produces a reaction of fear, unease and discomfort, because they're not sure how to deal with it. It'd be different if there was an anthology solely for polyamory or this FFM structure, but people weren't expecting to be hit with it out of nowhere, and hence the debates.
Personally, my issue lies more with the lack of development than the relationship itself, since it doesn't feel like an attempt to properly depict a polyamorous relationship and operates more like a 'solution' to the question of 'how do I make everyone happy?' Compare it something like Canno's work, which takes the effort to introduce the concept and put in good, solid foundations and development so that people can understand the appeal and dynamics of polyamorous relationships, and this story feels juvenile and reductive, especially with the, "Oh, polygamy was legal all along" part near the end. Of course, the author didn't have anywhere near as much space or time as they'd get in a serialization, but qualifying the polyamory from the very beginning rather than putting it in as a plot twist would've been a far better narrative decision. As it is now, the story suffers simultaneously from doing something that the target demographic didn't actively expect or want, and also failing to properly justify or establish why this new structure is just as legitimate as 'conventional' yuri. In short, good intentions, poor execution, and an unprepared audience.
