Clinton's 'I didn't say that, but if I did, it would have been a joke' is exactly what politicians say when they said something they don't want attributed to them but aren't sure if there's any evidence of them saying it.
Let me offer you another theory: she's almost certain that she didn't say it, but she couldn't remember every word she's said.
Which theory do you think is more likely?
Now I am not saying you are wrong, but you'd need external evidence to support your theory, until then, Occam's rule apply. If you agree that my theory is more probable but do not like it, then it's possible your current belief is skewed by your prior belief.