The five-minute grace period is not gone, but it has been recently changed. Information can be found in Shog9's answerin Shog9's answer on MSE. In this particular instance what happened was
- [a] comment has been added to the post since the previous revision by anyone other than the editor.
This ends the five-minute grace period, meaning a new edit will result in a new revision of the post.
The intention for this particular condition seems to be to make sure that comments don't seem completely idiotic when made during the grace period of the post which is then itself quickly edited: we can find out which revision of the post was commented on.