In the first place, you can't "retype" a memo and guarantee its accuracy. I've edited essay collections for publication, and in my experience even the most experienced scholars constantly misquote original material when you check the sources.
In the second place, the assertion that these memos are accurate, plus the assertion that these memos even existed in the first place, rests on the word of one man. After all we've been through with the press, anyone who trusts the word of a news reporter is an idiot.
So, we have only his word that these documents were accurately transcribed and only his word that they existed in the first place. You can even add a third possibility: that the sources leaked to him by someone in the agency with a political agenda were fakes. How can anyone prove anything, given these facts?
Exactly. If Michael Smith is lying, or if made an honest mistake in copying the memos, then they are fakes. But if he is both honest and accurate, they could well be fake nevertheless.