It is an expression originally evolved from corruption or deceit in American politics, which means evidence of a crime.
I'll stipulate to that, but then he says:
The expression is absurd in two ways. First, Saddam Hussein has already been caught red-handed
What does Hitchens think this expression means, if not evidence of a crime?
Is he saying that Saddam is more of a stabber than a shooter?
A smoking gun is evidence of a crime having been committed.
Sadaam has been caught red-handed and there is more than just evidence, there is proof of a crime.
Why would some idiot say we haven't found a smoking gun if we got the bullet-ridden bodies for proof?
He was making our case to not waste any more time...