The general way that this is used is "using only notes which can be found in one of the major scales".
I think most people would take issue with Brittanica about the pentatonic - the pentatonic scale is a subset of a major scale, and therefore is diatonic, at least in my reasoning. (Whole tone is not though.)
A diatonic instrument is only capable of making major scale notes (leaving aside things like note bending). For example, diatonic harmonicas (versus chromatic ones).
Diatonic harmony contains chords (or sequences) which again only use major scale notes. Therefor, a min 7 chord is diatonic, as it uses a subset of the major scale notes (even though it does not use all of them).
No, harmonic and melodic minor are not diatonic. Neither is a minor chord followed by its dominant seventh. For instance no major scale contains these notes:
C / D / Eb / F /G / B
which are the notes in Cmin and G7.