I noticed my error. To be fair, I don't think I read the diagram backwards. I think the diagram is drawn backwards instead. In it, iwd seems to talk to the iwd backend via D-Bus as they're close together.
Or maybe that's a diagram technique I'm just not used to.
It reads as a fairly normal diagram to me; NetworkManager has an iwd backend component/plugin that talks over D-Bus to iwd, which in turn stands on top of ell which in turn stands on the kernel (which itself contains a bunch of components of interest).
Or maybe that's a diagram technique I'm just not used to.