Looking at the JSON data in dev tools, it looks like there are separate `createdAt` and `indexedAt` fields, the latter of which was probably a later addition. For your—likely pre-migration—post, both are set to 9/11. On more recent posts, they're separate dates.
That post does indeed predate bluesky tracking index times, I remember seeing it before they announced that change. I believe it was motivated by other migration services becoming popular. Forward-dating was fixed even earlier, I think, since it might allow people to "pin" their posts to the top of reverse-chronological feeds.
Some of my favourite backdated posts are from the years 1776 and 1.
More specifically the reason we didn’t have an index time for that post was an architectural migration which lost our prior witness times. That was pretty early on. At this point we’d take pains to preserve those timestamps, and I’m fairly sure we will need to publish them for other folks to use at some point
