Probably because they are basing their discussion on the notes available in the key of C. A diatonic fretboard basically means that not all notes are present on the fretboard for one string, those are the wide spaces. If you tune the middle string string on your MD down to G, the third fret will be C. Checking all the notes on that string on the whole number frets will show that there are no sharps or flats.
When you base the discussion on the notes of the key D [G on the 3rd fret], the diatonic fret spacing produces an F#, the key of 'A' [D on the 3rd fret] produces a C# and F#. You only consider one string at a time.