The more I think about it, it does seem that Shiho has something like an incel mindset. She had feelings for Aki. Aki was treating her normally, and Shiho felt that she was entitled to more.
She was not. Shiho never felt entitled to more. She understood that her feelings were unrequited, and chose to distance herself from Aki because she knew she had no right to expect her feelings to change - and although they did change in the end, it was Aki's decision, one Shiho never pressured her for, nor even asked.
All Shiho asked was not to be forced to remain next to the girl that didn't love her back. She bottled her feelings because she already knew Aki had someone else she loved. She left the band, as she had every right to do, to not interfere with Aki. It was Aki who, despite Shiho having made clear that she wanted to distance herself, kept pursuing her and trying to remain close.
In the author's mind, Aki was in the wrong. But not for not giving Shiho what she wanted, and at no point did the story portray her not falling in love with Shiho earlier as a bad thing. She was in the wrong for trying to force Shiho into a friendship that she didn't want and hurt her. No, most people can't just "move on and remain friends" when the person they like doesn't return their feelings, and their choice not to remain around the other after they know their feelings are unrequited is not selfish, it's a basic right anyone has not to spend time around someone whose company hurts them. And I agree with the author that Aki was wrong, because I did the same when I was a teenager and I regret it.
Blastaar claiming that this is a reversal of responsibility is, quite frankly, shocking. What was Shiho guilty of ? Choosing not to be just friends with the girl that she fell in love with ? Whereas Aki would be innocent for pursuing someone who wanted to distance herself ? This take is, essentially, the friendship version of inceldom - the idea that you have a "right" to require someone to be your friend, even when they themselves do not want to.
At the same time, don't get me wrong, it's not like Aki made an unforgivable mistake. While yes, she should have respected Shiho's decision, she also didn't understand why she had chosen to distance herself. And although that's maybe not an excuse, she didn't realize that she was hurting Shiho in her attempt to remain friends. In the end, she also understood that she had been in the wrong, and only because she confronted her own feelings and realized that they had changed after Yori found happiness with Hima, and because she realized that there was a path where both Shiho and herself could find what they wanted, did she try to get close once again - successfully, this time. Frankly, I could not have asked better as a character resolution - as someone with good intentions, she made a mistake that was unintentional, she recognized it and regretted it, and she tried to mend it.