If he did use a computer, it'd be EASY to find out. Someone could go tonight and have a reasonable chance of finding out. We know the date the fax was sent from Kinko's because it's on the document. I used one of the Kinko's computers in North Carolina before. I had to show my ID and sign a log book. There were tons of names in the book from previous days. You'd just have to flip back to the day and approximate time on the fax and who knows what you'll find. Road trip anyone?
Here's another theory.
Mary Mapes was reported as going to Burkett to talk with him as part of this story right? What if on the way back to Abilene she decided to fabricate these memos? She stops at a kinkos, fabricates these things and then sends them on to Dan?
That would explain why there are some obvious errors about TANG stuff which you would think Burkett would not make?