You can indeed backdate records. Since the application knows when it has first seen (i.e. indexed) your record, it can decide what to do with that information. If there's a difference, the Bluesky app, for example, shows its own indexing time, but also shows a separate panel saying the post is backdated to some other date. Other apps could choose to show something else.

It is possible to create links asserting a specific version by making a "strong ref" which includes content hash.