Dave,
Keith, the line The wartime generals were not all cowards and incompetents can easily be interpreted as some wartime generals were cowards and incompetents. If Ms McMillan wanted to say none of them were cowards and incompetents then, surely, she would have said so.

Why would she spout such nonsense?
Sheffield said of British generals, "Some were incompetent, most were not."
I am sure he is right and that Macmillan would agree.

Has anyone ever claimed that any of them were cowards?
No.