Sort of like when Gehenna, Bluetspur, and the Nightmare Lands became Islands of Terror instead of domains of the Core — and of course, Markovia, which became a literal island in the Sea of Sorrows — after the GC.
Each of those moves made a certain amount of sense when the designers were trying to describe a cohesive continent that had developed inter-domain trade, travel, and diplomatic relations.
Those domains were all neat, and each one filled a certain niche of the setting. They just didn't necessarily work as well when placed next to civilized neighbors.
My sense is that the same sort of thing is going on here. By saying that each domain is an island unto itself ... the designers now didn't have to worry about the plot holes/inconsistencies that would be caused when one land was an apocalyptic wasteland ... while its next door neighbor was a renaissance marvel.
* Disjunction isn't really the opposite of Conjunction ... but I sort of feel like blowing up the continent needs a snazzy name. I'll leave it to the esteemed scholars of the FoS to come up with something more fitting.

[And I'm really looking forward to the FoS take on piecing these islands back into a cohesive Core]