When you edit a post within the 5 minutes period, the new edit is not considered a new edit, and a new revision is not created. If you just created a post, and then you edit it before 5 minutes are passed, the post appears as if you only created it. This means that the content of the "Edit Summary" field is not used; the field should not be shown in that case, as it is asking to the users something that is not even used.
I don't think the break in consistency would be worth the extra code here. It doesn't hurt to have the field present for rapid 5 minute grace period edits, and not having it might lead to confusion about why it is, and sometimes is not, present.