I think the logic literally is:
Person: "Yes, I wrote that usenet post"
Lawyer: "But do you remember what you had for breakfast that day almost twenty years ago?"
Person: "Uhh, no, I don't recall, it's a long time ago"
Lawyer: "So you're saying that maybe you don't recall writing that message at all?"
Person: "You must be right, I don't really recall anything at all from that particular day. So clearly I can't be sure that I actually wrote that post".
IOW, they seem to want to equate lack of personal memory of a particular date with some deeper existential doubt.
Lawyer: "Your honor, the witness clearly doesn't even remember the particular day they were born, so they clearly cannot exist"