Let us do a simple calculation. Then you will know why output swing can be much higher than VCC.
Suppose the impedance seen from the drain or collector of the transistor is 2 Ohm, voltage swing at the drain or collector is 3V. Also, suppose the efficiency of matching network is 66% and the impedance of terminal load is 50 Ohm.
Then the power output from the transistor is 3^2/(2*2)=2.25W. And the final output power on 50 Ohm load is 2.25*66%=1.5W. So, the final output swing is about 12V.