There's been a quiet change in their policy. The following is from a senior advisor with Apple Developer Program Support.
I am contacted you regarding the issue you encountered when attempting to edit screenshots. I have received an update from our internal teams and they have confirmed the behavior you are experiencing is now expected behavior.
Screenshots are considered “locked” metadata and can only be edited if the app is in one of the following states:
Prepare for Submission
Waiting for Export Compliance
Upload Received
Rejected
Metadata Rejected
Developer Rejected
Invalid Binary
So we are no longer allowed to remove or swap screenshots unless you first reject the application. You can swap screenshots at the time of submitting a software update to Mac App Store but will be stopped once you finalize submission.