Don't tell me the whole love letter development fell through a plothole... >:(
Could just be Onoda being slow to process the whole thing, she's the sort to struggle with and overthinkcarefully consider that sort of stuff.
That wouldn't be unreasonable. It's a trope you see often: someone confesses to another person (either by message or face to face), and the one who receives the confession doesn't reply immediately but instead asks for some time to think it through. Indeed, it happens a lot in manga.
But, if this is the case, I'd like at least some hints of awkwardness between them — not just interaction as usual, as if nothing was going on! >_<