The dilemma here reminds me a bit of the manhua "Straight Girl Trap." The basic premise of that one is similar, where a woman gets a crush on her boss and has to struggle to avoid falling into the eponymous straight girl trap- that is, the way that straight women can display intimacy with their friends in ways that, to them, are 100% platonic, but to a Sapphic person reads as romantic/potentially sexual. Hence, it's a trap you can fall into, thinking that they're into you when they're really not.
Add in cultures where being openly queer is still an incredibly dicey prospect, and I get why Hiroko would not only dismiss Ayaka's signals as being that platonic intimacy, but why she'd want them to be so. Hiroko is clearly attracted to Ayaka, but has multiple reasons (even setting aside spoilers) why she'd feel like Ayaka reciprocating would actually be a lot more trouble than it's worth. She keeps acting like Ayaka isn't sincere because, I think, there's a part of her that really wishes that were true.