I like that this chapter kinda mirrors/parallels the chapter how he said goodbye to them with the notes ("cheat sheet") and they went after him, even if it meant giving up so much they want him to continue being a part of their life even though his role is done, and in turn Fuutarou accepts in an awkward way telling them he's now just going to do whatever he wants to do and they'll now have to continue to put up with his selfishness, now giving them a connection once more beyond a tutor hired by their father.
And now Nino says goodbye to him with a note, and he decides to go after her, and as he implies by saying he's lonely, he still wants to be there for them/to continue their connection, even if it means he might/will lose one of the biggest things he takes pride in, his grades. And in turn, Nino replies as well, saying whatever she wants, complaining about him, and finally by throwing him a connection of sorts to them that will last beyond their student-teacher relationship, she loves him. I doubt she wasn't heard, Fuutarou showed no trouble hearing her throughout the ride and in no way does Nino look like she whispered or mumbled it.
I might be reading too much into things, but I loved the chapter, it was quite a sweet and emotional ride, literally.