Yes, the timestamp munging is an artifact of HN's re-upping system, described at >>26998308 and links back from there. About the timestamps, there are past explanations here: https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que....
The original times are always available because the modified time is used only on the frontpage and the article's /item page. If you find the article on /from or /submitted, for example, the timestamp will be the original. The two timestamps converge over time.