So its like one big striped RAID?

Its more like a proper Enterprise solution
1 Server for the database.
additional servers to handle the Business Logic..
you could easily expand the number of Online players using this model..
they may even choose to host different regions on different servers.. this would be the cheaper option in terms of complexity.
Segregating the regions by server could also mean you could control the amount of people IN one region.. so that you dont get a mass migration of people in one area...
If you had several 1000 people in Hydlaa square for example most peoples GPUs would over heat and explode...