That was the documentation with the incorrect nullability I was referencing.
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.
I doubt there is anything incorrect there. See the note here: https://wayland.freedesktop.org/docs/book/Message_XML.html#a...
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.