So - I guess the line here is Kusunoki had a friendship fall apart due to an unrequited confession.
I'd have to go back and read everything again to see if that was explicitly referenced before this point, but.
If that is what happened, then maybe that's her rationale with Keisuke: she's afraid their friendship would evaporate if romance got involved, so she's shut down the mere thought of it even to herself, to the point she looks clueless and ignorant of the concept whenever it's broached.
Or, taking the chapter where she meets up with those three girls as a clue, she actually is clueless about romance, but knows that it did ruin a relationship in the past, and so she stays away from it entirely, and doesn't really do any introspection into her own feelings (or, doesn't let herself).
Either way, Otobi actually having no real basis for liking Keisuke was actually more aware of her character than expected, because I too thought something big had to have happened off-screen for her to actually like him enough to be occupied about it.
Turns out, not the case. She might just have latched onto the first guy who was nice to her in close proximity, or she likes the idea of romance, as she said.
I personally think she should try going out with the the Confessor Friend. Even if it doesn't work out, it would give her some valuable insight into how she feels about relationships and dating and romance, and she's friends with him already; it's not unheard of to see that be a solid foundation for a relationship.
And that could potentially give her another perspective on Keisuke and her feelings toward him. Are they just surface-level? Is there something deeper she just hasn't hit upon yet?
Guess we'll see, depending on what Keisuke says next (because of course).