I think other people here missed the mark a bit on what Toiro’s conflict here actually is. They’ve been trying out stuff the think would make them look like a couple for the sake of putting up the fake relationship, but at the same time, Toiro also wanted to feel something akin to romantic love from these acts, and was depressed that she didn’t.
This does NOT mean that what she feels for Masaichi is not romantic, it just means that they’re not the type of couple they’re pretending to be. Pretending to be a typical couple didn’t work for Toiro because it’s not how they want to act around one another. But then why is she depressed? Why else but the fact that she actually wants to be in a romantic relationship with Masaichi?
Masaichi’s birthday speech hit the bullseye here. He himself felt like something was wrong as well. Acting like a typical sappy flirty couple didn’t suit them. However, he analyzed it correctly and knew what to say to Toiro. He didn’t say they have no romantic chemistry. He didn’t say to stop the fake relationship. He meant they don’t have to pretend anymore, not because they can’t act like a couple, but because there are also couples who act just like them normally. It is not invalid to view what they have together as romantic, so there’s no need to pretend to be something they’re not.
This is how he reaffirmed their relationship, as Toiro worded it. The fact that she was so happy hearing this means her insecurity is exactly that: She feared they can never be in a romantic relationship as they are. We know that’s clearly false, based on the copious amount of childhood friend romcoms we’ve all read, and now both of them do too.