Mullonde is entered through Orbonne via a portal, yes. But perhaps it's transporting you not to a pocket dimension, but just to another point on Hydaelyn? Mullonde Necrohol used to be part of Hydaelyn, was it not? There are ruins there built by man and there are undead there, implying there was once life, and a coliseum as well. Sort of like in FFT where Mullonde was actually on the world map in the middle of the ocean (via hacking the game, this could be seen) as opposed to being a pocket dimension. I think in both cases the portals just spirit you to a specific coordinate as opposed to another plane.