You nailed it. But he cannot possibly REALLY believe they are legit. He is PRETENDING that he believes they are legit. Why? Partly to keep the slime on Bush alive? And maybe to make it seem like he is innocently an idiot?
Well, I've wondered this... Dan is 72. It's very likely that he's not familiar with computers too much and wouldn't realize how much this document has the characteristic look of a Microsoft Word document. It seems glaring to me, but I use Word almost every day.
However, that said, Dan is 72. Being about 40 at the time, he would be very familiar with the characteristic look of a page written by a manual typewriter in the early 70s.
How he can say he didn't, er, make that doesn't, have any suspicions about these memos, is beyond plausibility in my opinion.
If he admits that they're forgeries, he cannot claim that his source is protected by journalistic confidentiality.