He wrote it. After he was fired. The computer would prove when the document was created.
Wasn't everything in Comey's office seized when he was fired? Comey said he wrote this memo to protect his concerns in the future. If it isn't on his official computer, and he didn't immediately note it and put it as part of his papers, how did it even exist?
Even if he wrote the memo then destroyed all copies (doubtful there ever was one) he has no way to prove when he wrote it. Thus, it doesn't exist.
It's either perjury or a serious violation of procedure.