The sentences "They're yuri", "It's yuri", "She is yuri to X" are such a nonsense and mostly put aside the use of the word lesbian in yuri manga, come on lesbian it's not an insult, and lesbian relationships.
The problem shows up with how we can't know if Mika was making a statement at the time about the terms yuri (fiction) versus real (lesbian) sapphic romance, rather than a statement about yuri versus doubt that relationships between women can be romantic. Reading this chapter I realized that Yoshioka would have learned a lot about her sexuality through online sources outside of yuri fiction and otaku culture, so realistically her self-perception would gravitate more to the term lesbian and less to the marketable "yuri" through which Fuyu learned about her own sexuality.
If I'm understanding right, Mika was in love with Fuyu when Fuyu was working up to a confession, but Mika hadn't labelled her own feelings yet? So Mika's mistake was she rejected the idea of romance with other girls because she really did believe it only happened in stories, and took offense because a "yuri" fiction label or an assumption that romance is present would devalue what she considered a close friendship? Though Fuyu seemed pretty open about being a yuri fan she wasn't mentioning it directly when she was working up to confess, so I get the impression Mika hadn't come to terms with her own feelings at that point. Eventually Mika realized that Fuyu was distancing herself because of "the kicker", probably even that Fuyu interpreted it as rejecting her and not just demeaning her genre. If not while the two were growing apart in middle school, Mika respected her own feelings by the time she sent a text at the exact moment the mythic lovers supposedly meet on Tanabata earlier on. That's a bit tragic.
If Mika already knew her feelings were romantic (lesbian), and answered Fuyu while making assumptions about Fuyu's connection with yuri (imaginary), and still let the relationship crumble, the development is more of a forced misunderstanding.