This is something that gets needlessly convoluted:
Diatonic simply means "belonging to a specific scale". My only references are the ones you've already posted, it's just that the definition gets skewed.
So B is diatonic in the key of C major, but G# is not. Eb is diatonic in the key of C Dorian, but is not diatonic in the key of C Lydian.
Whether a note is diatonic, therefore, depends on the note and the scale in question. You can't answer the question without both pieces of information.
