"Just as in the core D&D cosmology" literally means they are similar but explicitly not the same.Since Kieth's comments were mentioned, I thought qouting them here makes sense.
I know about a theoretical connect via the Plane of Shadows, did not know about the connections with the other transitive planes. Although there is no Plane of Shadows Transitive Plane outside Eberron anymore so it's like what is transitioning between now. It's like an Orphan Plane.
Also Eberron being impossible to reach or level is out the Window with Vecna: Eve of Ruin. Even E: RftLW had Vi in Sigil as a Planeswalker.
And what is blocking one from going through the transitive planes to reach Eberron, the Crystal Sphere? Does it exist in all planes at once?
Anyways back to Celestials, it does leave the Question of out Titans/ especially Empyreans if both types fit into the setting as well if they are made in a different way to the rest of the Multiverse?
It's a retcon.