@Lorpal, you are correct that this is how the Diatonic algo works, and it does shift the intervals according to the selected key.

However, the Harmony Man is adding different intervals to some notes. In the case of C maj for example (when choosing the high voicings) it adds the 3rd up and 5th up when the root note is played, but for the rest of the notes it adds the 3rd and the 6th. It also changes when using the low voicings – when the root note is played it adds the 4th down and 5th down, but adds the 3rd and 6th down for the rest of the notes.