If a contract is not having fallback() or receive() , it simply means it won't accept your ether, the easy way. [ ofcourse there is always a way if you know ;) ]
Now because in King.sol there is a receive() which has to go through a condition where previous king will be sent the msg.value but if previous king aka contract doesn't accept the money, game breaks here itself~!
Here is another implementation of DOSAttack, a few repositories back we also went through one.