You got it—that's exactly what's going on. Instead of inviting a repost, we're re-upping posts by resetting their timestamps and otherwise treating them the way an invited repost would be. So far we're only doing this for submissions that are up to a day old, but I don't see why it couldn't extend back a week or two, especially since that would let us privilege the original submission in most cases.

We considered resetting the timestamp a while ago and thought the community wouldn't like it, but people turned out to dislike the repost solution more, because it pollutes the story stream with more dupes. A couple users suggested the timestamp approach, so we gave it a try, and indeed it works better. It also has bonus properties like not relying on an account having an email address (or people reading their email).

This is the latest in a series of experiments we've been running to try to give the best stories multiple chances at making the front page. The holy grail of this is figuring out a way to distribute the work of picking 'the best stories' to the community, in a way that doesn't just reduce to upvoting. (The latter bit is important, since if upvoting worked for this, we wouldn't have the problem in the first place.) We're hoping to get to that soon.

Edit: I forgot to mention that the re-upped timestamps only appear on the /news and /item pages. If you look elsewhere, such as the /submitted page for the user, or the /from page for the site, the original timestamp is displayed. So you can think of /submitted as the historical record and /news as displaying a timestamp relative to other posts on the front page.

Edit: here are some links for anyone interested in how these experiments have evolved:

https://news.ycombinator.com/item?id=10395389

https://news.ycombinator.com/item?id=9866140

https://news.ycombinator.com/item?id=8790134

https://news.ycombinator.com/item?id=9828818