The reaction to this latest chapter is honestly just embarrassing. Reimu didn’t show “uncertainty” and Marisa didn’t “fumble”. She confessed honestly, knowing rejection was possible. Reimu clearly shut it down. That’s fine. Calling it a “fumble” or framing it as Reimu being unsure just reveals how many people don’t see Reimu as her own agent at all. She’s treated as a prize Marisa has to get to, or some idiot that’s just not aware of her feelings. Not as someone who can simply… not reciprocate? Especially since she never had a clear “I love her” moment like Marisa did? What really pisses me off is that the story itself set all of this up too.
This feels like a somewhat bad-faith reading of the reaction. Sure, in a realistic situation, you can read Reimu's response as a normal rejection of the confession. From a narrative standpoint, though, we know this is a ReiMari series and have seen Reimu struggling with her own feelings and wanting to keep the status quo despite them. That is, in my opinion, why she won't even let Marisa finish her confession: Reimu is unsure if she can reject her properly.
And, to that front, Reimu's response is a fumble. Denying Marisa's desire to get her feelings out already makes things difficult, but then she tries to stop Marisa from leaving, really trying to drive home the idea of just burying it and remaining friends, but without giving Marisa the space she would need to sort out that outcome. (Honestly, I think Marisa was even prepared for Reimu to just reject her feelings. The response she got was the worst possible outcome for where her mind has been these last several chapters.)
Marisa also fumbled by trying to make a big show of her confession. She did all that work to make it a special moment and left herself all the more vulnerable to the heartbreak. All these factors combining is why the ending is such a knife to the gut emotionally.