The beauty of a memo that might not exist is that failure to produce it and prove innocence can be taken as a sign of guilt.
(Yes, we're there, as a country, where the presumption of innocence is no longer SOP).
If produced and not damning, it will be called "a fake".
If claimed not to exist there will be claims of a "cover-up".
The whole issue is of the nature of the question: "Has he quit beating his wife yet?"
It's the FBI. 30K emails (known), ALL illegal. Over 200 Code Word Top Secret Files, all on a private server, and they could not come up with a chargeable crime, much less a conviction. Q.E.D. Incompetence.