I remember seeing leaders on this forum editing posts in the last version of this forum (discourse). It is also stated in the discourse official blog that leaders are capable of editing all posts.
This can only mean that:
- Admin tweaked the settings.
- You failed to find the button.
Honestly, I don’t see a practical reason to restrict editing while leave merging / closing / unlisting open.
Sometimes you really have to point out where the error is or else it just becomes invisible 