Well, it's not like a whole lot needed to be said.
Natsuka being there and calling him by his name meant Seno won Natsuka's heart. Especially, when he was given that smile.
Plus this wasn't but maybe a few days after coma guy woke up, Seno was drowning in depression, and he wakes up in the hospital and he's confused at how everything he did was brushed off by Natsuka but he goes with it.
The moving out part with all 3 of them showed him how nothing he did phased Natsuka in a way to push him away and coma guy knew all about everything and wasn't being judgey about it at all. It was like all 3 of them were friends for years.
If you have full communication with no misunderstandings then everything just clicks.
I was hoping for seno to take a little more time to realize that the one natsuka likes was him. Because he was so emotionally vulnerable and plus not being able to believe he's the one until later (like natsuka convincing him with more evidence and confirmation) suits his personality more. Or maybe I just want the angst to last longer lmao so that the manga can be wrapped up more beautifully and happily when they love realize their mutual love. After the hospital scene, suddenly everything sped up too quickly imo.