I think it would be quite valid to define "in focus" as "where the expected image looks right on the ground glass", essentially it can be "focussed properly for the task at hand".
I think her point is that a common/shared/scientific definition isn't a requirement of good photography.
In context Ms. Cameron is one of the people who helped start the soft focus era in photography.
In the paper I referenced above they had an interesting discussion about focusing the lenses of her day, her choices, and her possible failings; seems the chromatic aberration on many lenses required a normal movement of 1/40th of the focal length to fix focus, after you focused. It was not a WYSIWYG world as it is today.