I agree with the Encyclopedia Britannica in restricting the definition of diatonic and chromatic to within the Western heptatonic scales.
The way the Wikipedia article opposes diatonic and chromatic in different applications also makes sense to me. This distinction evolved at the interface of when melody gave rise to harmony in the Western context, as key-consciousness became a factor, marking the start of the Common Practice period in the early Baroque era.
I would say that it is instructive to conceptualise this as a continuum, from the purely diatonic like 1st species counterpoint to heavy late Romantic chromaticism. Beyond these borders, pentatonic and modal tonality (with their absence of key-defining leading tones) past the diatonic border, and whole-tone harmony and atonality at the chromatic end.
Hence, within the boundaries of "Common Practice" in the Western canon, there are varying definitions of "diatonic", which the Wikipedia article gives a good rundown of. All agree that:
- major scales
- natural minor scales
... form the bedrock of what it means for an interval, a melody and a harmony to be "diatonic". Where they differ is how "diatonic" the other forms are:
- harmonic and melodic minor scales;
- augmented fourth chords and half- and fully diminished 7th chords, which arise from these extended minor scales;
- other chords with accidentals outside the key (Neapolitan 6th, secondary dominants, chromatic mediants).
So different authors do have different definitions.