@HertzDevil, thanks for pointing this out.
We originally posted this change to enable some examples that we are preparing for our second theory book, Hooktheory II. Since then, we’ve realized that labelling accidentals on the add9 is a little more delicate than we had originally thought.
The issue really has to do with the fact that we are merging Roman numerals, a classical theory device, with (add9), which is really a popular music concept (same issue with sus4 as well). The question is whether it is more useful for the “9” in add9 to refer to the interval with respect to the root of the chord (in this case, a major 9th could be add9, a minor ninth would be add♭9, and an augmented 9th would be add♯9), or should the “9” refer nominally to the note that is 9 scale degrees up from the root, in the given scale (in this case iii(add9) would be understood to contain a minor 9th, since that’s the interval that is made between scale degrees 3 and 4 in the major scale).
From a purist perspective, it would seem that the latter interpretation is more correct. After all, we don’t qualify the figured bass 642 with accidentals depending on the quality of the 6th and 2nd intervals. The only way we know that the “2” in V42 is different from the “2” in I42 is that although both V and I are major chords, we recognize that that 4 → 5 is a whole step and 7 → 1 is a half step. This is not dissimilar from recognizing that the difference between a vi(add9) and a iii(add9) is that 6 → 7 is a whole step but 3 → 4 is a half step. In Hookpad 2 we are discussing generalizing the theory to work with arbitrarily defined scales, and would seem that needing to qualify figures relative to the major scale in some sense hinders our ability to try and internalize a new sonic space that is provided by a nontraditional scale.
However from a practical perspective, many people think of (add9) as part of a quality. Along these lines, one would expect minor chords embellished with (add9), for instance, to always sound the same, regardless of what the root scale degree is. I can certainly sympathize with wanting to know that a ♭♭VI(add9) and a ♯III(add9) have the same quality without having to figure out the context of how that chord came to pass.
Ultimately we haven’t finalized this decision, and appreciate feedback on the issue.
Ryan