That sounds exactly like the old Morrowind loading system. It was relatively seamless at the time for traveling about outdoors, sans the mini loading between these cells. It used to take a few seconds back in the day it was released but with modern SSDs it takes a fraction of a second.

Oblivion and Skyrim made this for the most part invisible. But loading times for indoor transitions still existed.

I am curious and exited to hear what tricks they have used.

Theres nothing in the engine reqiring to fill every cell between two cells. Might as well be empty space, or cells with random generated terrain.

Also how are they doing the ship interior cell? Loading a cell without removing the old? Why aren't they using the same thing for interior windows in houses then?