As someone else in the thread said, it could have been ported in from another site.

Also - if something was originally posted in a Gift Exchange, and didn't get revealed 'til the Exchange opened, it would be way, way, way down in the feed by the time anyone could look at it. It's considered acceptable to redate a fic to the time the Exchange opens (or, some people wait until it's off anon).

I would strongly advise *against* redating a fic outside of adding a new chapter or the above situations, though.