I suppose I interpreted it differently - in the previous chapter, Nobara accused Ria of thinking she's confused for loving women, and Ria replied "You are you, I am me" (emphasizing their differences). Combined with the fact that Ria apparently hasn't dated a woman in years (at least according to what Nobara knows, and they are close), I thought she might have realized she was straight.
But indeed, it's possible that she is indeed bi and just hasn't found a suitable partner in years. We at least know that from Nobara's point of view, it appeared as if Ria was "siding with" her parents and justifying their homophobic view of "it's just a phase". Her statement was still wrong even if her interpretation had been correct, she shouldn't just assume that every straight person is homophobic... but then again, she feels isolated, she doesn't have anyone to confide in, and the one person she was looking up to appeared to her as validating her parents' opinions.
And in ch. 204, Ria talked about "treasuring the person she liked at the time". She did not say anything about still liking women... and worse, that sentence can (should not, but can, and Nobara wasn't in the right mind to see the difference) be interpreted as agreeing with Nobara's parents that it was a phase. You can see Nobara's mind shifting tracks to believing that in the last panel of that chapter. Despite the topic being brought up, Ria didn't contradict the parents... and because of that, they continued spouting their idiocy about "becoming proper" and "youthful folly" in front of their teenage gay daughter.
But importantly, through all that, Nobara didn't say anything bad about Ria being with men. She didn't judge in any way her orientation, only accused her of agreeing with her parents. I understand why people would view it as biphobia, but I would interpret it with more nuance, as Nobara has not said anything negative about being bi.
The point isn't saying that Nobara was right, obviously. But from what she knows, an accusation of "Aren't you agreeing with my parents' homophobic views ?" is reasonable enough, because she has little ground to believe that there are people supporting her around her. I don't find it acceptable to equate a girl's lament about being surrounded with homophobic people, to the oppressive homophobia that other people have expressed around her.
Both wrong, but not the same. Which is why I take issue with Ria's reply, and I guess I should emphasize this because it's my sore point with this chapter (despite liking the previous and next ones). She can legitimately be angry at Nobara assuming she would be homophobic. But due to the parallel she's drawing, she appears to be finding excuses for Nobara's parents as well, and they certainly don't deserve it.
Nobara would certainly have felt less depressed and isolated if she knew Ria still supported same-sex relationship, unfortunately, they never had that discussion... I know it's hard being queer when you don't have anyone you can talk to.
Edit : In fact, now that I rethink of it all, most of Nobara's doubts and self-hate came from that. She kept hearing from her parents that being gay was just a phase. And then, for the one older person she was looking up to and who didn't make her feel isolated... it appeared to have been a phase, and the topic of bisexuality was never brought up. Even if she didn't want her feelings to be invalidated, seeing as Ria had chosen a man over a woman, it made her doubt the viability of her feelings for the future. That's why she broke up up with Hiromi, because she thought she would be an hindrance in her future. That's why she broke up with Io, when the future was brought up and she thought their relationship couldn't last.
This is depressing. For something that is mostly comedic, I didn't expect to overthink about a character whose sense of relationships was broken by being constantly exposed to homophobia as a child and teenager. Fuck those parents. It... just hope this has an happy ending for her, once she realize that yes, gay people can have lasting relationships and fulfilling adult lives.