Oh, I don't think that value is believable (hence the 'somehow'), but, I don't know how you'd get that value so I'm basically saying 'anything is possible apparently'.
I'm not entirely sure how they'd have that account age either, but it is worth noting that the Fediverse itself (or something in this vein) is much older than either Lemmy or Mastodon (to address @animist@lemmy.one's point), so even setting aside database futzing it may not be impossible.
One of the older forms of this that I'm aware of (besides Usenet, which is running on different tech), is OStatus & GNU Social, which interestingly enough may be hitting that 10/11 year mark (or older, wiki page on the subject is unclear on when it originated).