I haven't listened to the podcast because they warned about Dawnguard spoilers I'd rather not hear about yet.
If they're talking about cells and object count, my guess is they're talking about the memory limit on how many things you can stick in an interior cell before you run into crash problems. The default for this is absurdly low, something on the order of a mere 1,000 objects. Larger dungeon cells push these limits easily. Exceeding this limit in an interior that connects to a worldspace almost certainly means a CTD when the user tries to exist. It's let some people to have to use buffer zones between the main body of their work and the rest of the game.
There's an ini setting that eliminates this problem:
iLargeIntRefCount=999999
Sticking that in the [General] section of Skyrim.ini has been shown to eliminate the problem. The game would probably choke and die long before getting close to 1 million objects in a single cell.
Worth noting this problem only exists for ESP files. ESM files don't have this. I can't see why Dawnguard would have had this problem unless it's coming out as an ESP.