It has unique IDs for both entries and comments. I'm not sure how it generates these. Comments, at least, are clearly generated server-side, because it lists deleted comments (although not their contents.) And there's no way it can know about those otherwise because it lists really old ones, dating from near the beginning of my journal. (It does, oddly, list usernames, which surprises me, but nothing else. This, incidentally, is why I know they're genuine.)
Entries might be serverside, or they might not be. I'm honestly not sure.
In any case, I recently wrote the entry that it thinks is Entry #3000. It's this one.
I could not have planned that better if I'd tried.