Thanks for asking about the harp(s) I used on that.
True, I didn't create this tune for reasons of nerdiness; the means I used were just the tools that created the set of potentials that made this particular tune possible.
The answer to your question, though, is kind of complicated and nerdy:
The background harps you hear are chromatics. But the foreground one playing the main melody and chords is a pair of diatonics in a Brendan Power Switch-Harp assembly that lets you yoke two harps together and switch back and forth between them with a slider assembly similar to that of a chromatic harmonica.
But that's not the complicated part.
The two harps are in a Magic Harp tuning that plays a pentatonic scale, with the result that the chords are voiced mostly in fourths. I call this pentaquartal tuning, penta for the five-note scale, and quartal for the quartal chords (i.e., consisting of intervals of a fourth).
For instance, a C pentaquartal harp migh start on E as blow 1 and then go:
E A D G C (which is the C major pentatonic voiced all in fourths, and then repeat E A D G C again. At the point where th pattern repeats, you do get C to E (a third) and you also get G-C-E (a C major chord) as well as C-E-A (an A minor chord). But everything else is this stack of fourths. (The draw notes are the same note sequence, but starting on G as Draw 1.)
The two harps in the Switch-Harp I used for this are in D and Bb pentaquartal tuning, with both having the starting note of D below Middle C in Hole 1 Blow. That D (the only note in common between the two scales) gives the common top note in the alternating D and Eb chords in the intro and outtro.
The chord progression on the main part of the tune is D - G (played mostly on the D harp), to G minor, Bb, and Eb (played on the Bb harp) back to D (back to the D harp). The intro you mention is just D and Eb chords alternating, using the D harp for D and the Bb harp for Eb. The interlude, mostly guitar with chromatic in the background, is Bmi7 - G - Eb - Ab - Asus. I could have handled that with the pentaquartal combo but chose to give that to guitar for a breather from out-front harmonica.