Its less about how clients exchange data.
Such a modes can run peer to peer or server controlled.

Its more about how the game-world is calculated.

In a classic client-server game, the server does all the
logic, calculations and oversees validity.
In the "syncronous simulation", each client is calcualting
the world logic, only based on usercommands from
all clients.
The server (such as battle net) would just handle
the connections and transfer of the commands.

I know its how Starcraft 1 worked (looking at the networking
commands). So I assume SC2 is very similar to that.