I don't remember going to the restroom and taking a **** a few years back, but it's a pretty good bet that it happened. She knew what she had done, and that answer was what is called an equivocation, a lie. If it is untrue, it is still untrue. The equivocation part comes when she tells a lie that is impossible for anyone to prove - that's called trying to prove a negative. She is very talented at such speech, she is an attorney for goodness' sake. This ranks right up there with "it depends on the definition of IS" and "I did not have sexual relations, with that woman ..."
We cannot prove that she did not forget. She knows that. That's why making that statement is chosen by attorneys to have their clients use on the stand when asked a question that the truth could incriminate them, or at least make them look less than innocent. There are a few variants of this answer - "I do not recall." "I do not remember making that statement." "I don't have a memory of that." "Could you repeat the question?" - in hopes of the question being reworded to allow for one of these type of answers by parsing words.
If the document had not been found, then I would say that's it's possible for her to not remember. But, she has shown that she has detailed memories of every action she took that she feels would exonerate her, but has a marked loss of memory regarding all actions that could implicate her in breaking a law.
Down here in the south, we call that horse ****.