Scenes like this come about because I find myself with no clear way to move the story to its next intended point and then realize I have two or three plot threads that haven’t been acknowledged in a while. Imagine you’re building a bridge out of Legos but the pile of bricks you’re using runs out of long pieces to make a span, and while wondering “can I build this section with a bunch of little pieces without it collapsing?”, you remember there’s an old Lego kit in a box in the closet you haven’t played with in years, but you know it’s got a bunch of those long pieces you need.
Maybe that’s not a great analogy, but anyway, Noah’s regret for his role in Flint’s latest transformation hasn’t been brought up since Part I; Flint hasn’t flown in two chapters; and Cass and Flint haven’t interacted AT ALL in this entire story arc. It didn’t take long after realizing all of this to come up with a way for the three to come together and formulate (or at least argue over) a plan. (Personally, I think Flint makes the best point, and he doesn’t even say anything.)