FBI memos are notes kept by agents of calls they have with people and are admissible in a court of law as evidence iirc.
edit: Calling the memo a lie would require an admission the memo exists. While it's obvious that it does as per reporting by multiple agencies they see it easier to shriek "fake news".
This just goes to show they know they are fucked. This is the first thing that they have clearly not tried to spin at all just said "it's a lie".
If you spin, you can at least say "I didn't lie" later. But there's no coming back from this one.