I'm starting to wonder if when it was first being developed, the makers had plans to make it akin to D1 level 5, then for some reason changed their minds and took a different direction, and the one reference in the objectives got overlooked and not adjusted accordingly...
I heard somewhere (not sure how reliable a source it was) that the Fusion engine only loads rooms adjacent to the one the player is in (presumably not counting doors, which would explain why they have their own room type instead of just an object type). And there's probably something in place to calculate whether the player's line of sight passes through more rooms at once but still... it would mean the actual size of a map wouldn't be much of an issue for CPU load.