So apparently X only allows edits to the text of root posts, not replies, not media, not private posts.
And it only allows edits for one hour after the post is sent, and only up to 5 edits total.
https://help.x.com/en/using-x/edit-tweet
I think X must have zero users now because they insist in not implementing full-blown unrestricted capabilities for such a crucial unnegotiable feature.