Some writers use upper-case Roman numerals to indicate the chord is diatonic in the major scale, and lower-case Roman numerals to indicate that the chord is diatonic in the minor scale.
From the en.wikipedia.org
For this purpose, the harmonic form of the minor scale is considered the tonal-harmonic basis of its diatonic system.
From the en.wikipedia.org
The most familiar of these are the two diatonic scales of seven notes each, called the major scale and the minor scale.
From the en.wikipedia.org
By that reading, the definition of diatonic scale is not anomalous, but includes all standard forms of the minor scale.