James Comey personally can introduce his copies of the memos and testify to their recorded information. This helps insulate him against charges that he is misremembering the time of the statement or the words that were said. They're still reliant on his testimony that they were accurate when he wrote them, but they protect him against incorrect recall.
Comey sent the memos to other people. The other people can testify as to when he sent them and what they said at the time. Also that their copies weren't tampered with from the time that they received them.
Comey sent some of the memos through official systems, e.g. by email. The times, size, and possibly the content of emails sent to and from government addresses can be reproduced after the fact.
Even if Comey had official records, they still wouldn't supply anything more. We have to rely on his testimony as to what was said and when. Any records only show that he made that claim at an earlier time. They don't show that the claim was true.
You don't include the exact wording of the CNN statements that made you think of these as official records. The quoted statement does not indicate that to me. The one thing that the memos establish is that he was making this claim to others previously and that he wrote down the words shortly after he would have heard them. They don't need to be "official" to establish those two things.