I think the key to understanding the idea is to separate the concepts of tonal and modal in regard to scale degrees and functional harmony versus modal color.
In a nutshell, the tonal scale degrees are
^5, and the modal scale degrees are
^5 provide the roots for the all important tonic, subdominant, and dominant chords, while
^6 provide the thirds for the tonic and subdominant chords.
^6 can be either major or minor and so define the mode of a key.
To some extent you could say scale degrees
^7 have mixed roles, but in regard to functional harmony the raised
^7 degree, the leading tone, is critical to dominant chords and it's "function" is to define the tonic in both major and minor keys.
Another way to say this is: the scale degrees
^5, and leading tone
^7 do not vary between the major and minor forms of a key. The functional aspect of harmony doesn't change between major and minor forms of a key:
^5 goes to
^1, leading tone
^7 goes to
^4 goes to
^3. That's the essence of functional tonic/dominant harmony. The variable major or minor aspect of
^6 only provides "color", determines the mode, for a key.
A common convention in harmonic analysis is to use upper case Roman numerals for major chord quality and lower case for minor quality, like
I for a major triad and
i for a minor triad. But there is also a convention that does not distinguish major/minor quality and only give upper case Roman numerals to indicate chord roots. By that second convention
IV V I only means "subdominant, dominant, tonic triads with no regard to modal quality.
So, we can define harmonic function without knowing the details of mode. You can think of that as either modal ambivalence or as accepting a much more "colorful" harmonic style. I suppose if you push that idea far enough you could say there really aren't separate major and minor tonalities but simply a single tonality. There is no name for it, except perhaps "tonal harmony."