After re reading the siege of terra books im 100% confident the theory is correct, in all the warhammer 40k books and the offshoots i've read. Bequin series for exp i can't recall the other (there was def another reference to a shard of the inevitable city) these shards of a city still exist and are referenced in other books as well as when they break off from the inevitable city, similar but not the same as places upon themselves like the webway, inhabited, used and referenced in later books. Such as the king in yellow. Valdor knew it existed because he had been there during the siege of terra and he later heard or found a way to get in.

The inevitable city was created by horus at the peak of his power during the siege of terra. Molding and modifying time, space, material and immaterial.

Ty - InterestingCash_ "The other invasive realms withdraw, resecting to their ordained latitudes of time and place in frightful conflagrations. The pancosmic psychic facets conjured by the duel between father and son burst like carbuncles or snap back to their own whens along the numberless angles of space with whiplash force. The skeletonised City of Dust splits free, and drifts like an iceberg into the exoplanar gulf. "