There are two places to morph a creep colony for each overmind. I studied the one which is harder to defend when you do a wall-in.
Here is a result.
The red circles represent a position where you can click a creep colony.
As you can see, 6-7 SCVs can simultaneously attack a creep colony, but
when you do a wall-in, only 3 scvs can attack it from your side of the wall.
The time required for 3 scvs to kill a sunk is roughly equal to the time for a full health creep colony to morph into a sunk. So you can surely kill it if you start attacking it before it starts to morph a sunken colony.
You have to send scvs to a correct position before you give them an attack command, otherwise, they often fail to find the position to attack and just stop.
Now you can defend sunk rush!