I think it just comes down to that they just haven't fully worked out all the details of Ilsabard yet, although the texture graphic of Hydaelyn as seen from the moon clearly shows Ilsabard's basic geography and coastline now, SE probably haven't yet finalized where all it's towns, cities and Imperial provinces are arranged and located, which may or may not be added as future content. Hence the Three Great Continents map still shows the continent mostly obscured by cloud.

They'll probably get around to it eventually, but for now the clouded mystery of Ilsabard's interior on the map remains.