wayland.app just HTML-renders the contents of the specification XML files. If a compositor or client is not interpreting nullability the same way wayland.app says it should be interpreted, then that's a bug in the compositor or client.
What if no compositor is interpreting it correctly? We tested on Weston, GNOME (this was shortly after it went Wayland-only) and Plasma and the same problems persisted on all 3. In all fairness, the exact functions with wrong nullability might have been different, I don't remember, but the nullability issue itself was persistent on all 3.