How do I add a commit to a parent branch in a stack?

[x3ro]

There's a section with the same title in the README, but I'm somewhat confused as to the meaning of the following:

If this is to append to the parent branch, for now you'll have to use git rebase -i

As an example, I tried switching to the parent branch of a stack in one of my repositories, and added a commit there. Then I ran git stack --rebase, and the stack was updated correctly, without me having to use git rebase -i.

Is this expected? In which cases would I have to use rebase -i in addition to git stack --rebase?