What do you mean by "those" resistors? Do you mean the 3 1K resistors that go from node A to node B? They chose them because they are the ones that determine the resistance between node A and B once the current sources have been deleted. Since the 2k resistor has one end not connected to anything, it doesn't have any effect on Rth.
To find Voc, you can use any method of solving the network. If you can determine the currents in each of the 1k resistors, then you can multiply each current by 1000 and add up the voltages across each 1k resistor, taking into account the polarity as you follow the path from node A to node B, to get Voc.
