Apparently they at least have distance based culling on actor replication, which is more than I thought:This is why I'm going to be interested in seeing how Pantheon will fare once they scale up their testing.
I do know that Epic has been trying to hire senior engineers with MMO experience; I'd assume it's to help address some of their networking issues when it comes to larger player counts.
https://wiki.unrealengine.com/Replication