I get what you are saying, but after reading 100s if not 1000s of manga in my life I can say that 90% of the people who engage in discourse do not read that deeply into it. So many people just want to go against the grain, and choose to just like a character immediately, or ship a character immediately because its not the lead. Saying its "boring" for the lead to win when the story is about them.
While I see what you are saying, but I would argue that neither of them are handling his phobia well. One of them disregards it (not as much recently). The other essentially coddles him. Neither are good necessarily, but one can lead to helping him get over it, and the other keeps him trapped and essentially isolated. I would even say that her keeping him comfortable is kind of a bad thing. Because it just leads to him being ostracized by everyone.
Notice the difference between how they both behaved.
Nishino didn't try to stop him from going to the festival with his CHF. She was bummed out about it, but she didn't try to get in the way.
Meanwhile CHF is trying to keep him isolated from his new friend by shouting "Don't leave me", and then picking the worst time imaginable to confess.
I personally feel more bad for Nishino than the CHF because she had to hear that. Now its just going to be awkward.
A sizeable portion of people having nothing more than a shallow level of engagement with a series is certainly an unfortunate reality, but being dismissive in the other direction isn't solving that problem when it just serves to further muddy discourse where it's actually meaningfully occurring.
And no, neither of them are "handling the phobia" well. But that doesn't discount that both are important for the MC and his situation, as well as representing different stages in his life.
More to the point - I think the problem that at least some people have, though, is that Tsuzuri is being reduced to a plot device to show The MC moving on from his past--but it involves hurting a girl who only wants the best for him and to be happy with him, and it kind of just sucks to see her get introduced, be
likeable, and then shot down and left heartbroken because there's just no way the MC can move backward in time. It sort of removes her agency as a person in the setting, and sets her up to fail by default the moment she appears in the narrative.
And when she's a cute girl who wants to help, having her be a by-design losing heroine that is necessarily rejected by virtue of the MC needing to grow will rub some people the wrong way, as it just kills her character. Maybe she'll get something after this, but she's here to serve a purpose in service to Nishino right now, and nothing more.
Beyond that, you're ignoring the context of Tsuzuri having long-standing feelings for him,
and him very recently soft-rejecting her by calling her a close friend of importance. She's reeling from being friend-zoned and, I would argue, is taking this chance to at least make the attempt to speak her mind, before she loses all nerve.
Plus, he'd walked away from her once already, effectively shutting the door in her face on continuing their relationship all the way back in grade school.
This moment represents a repeating of that history between them, and it would effectively mean she's lost. This reads to me as Tsuzuri's last-ditch effort to make her feelings for him known, to step up and fight when she couldn't do it before.
Nishino has also had a history of dragging him along and doing all manner of things without consideration for him. It's only been
very recently that she's shown any restraint at all and actually deferred to his wants and feelings on a given matter. That is commendable as a bit of character growth for her, but if anything Tsuzuri is doing what Nishino would have done if this were still in the first 10 chapters of the story. She might not get another chance if she doesn't speak her mind here and now.
And, it's not clear that she's aware that he hadn't yet ended the phone call, nor is it clear that she is acting with the intention of Nishino overhearing them. It's certainly unfortunate that it's playing out this way, but I don't think you can point and say she deliberately sought to stop him from terminating the call before speaking.