Yeah, it seems fishy to me that they changed - especially because the 'sometimes inappropriately extended to' seems to refer to their earlier edition.
They might actually be basing the new definition on Alberti's discovered scores. I read that some were found (six as I recall) in the early 90s, so they may have influenced the '99 edition.
And yeah, I guess quasi-arpeggio is fine. It's just that the idea in Bach is melodic - you wouldn't call a major scale a maj13 arpeggio. The preludes give a perfect example - #1 in WTC1. It sure sounds like an arpeggio with that C major triad opening... but it's not, when you look at the piece as a whole.
There's a melodic pattern that exists throughout the entire piece (up until the last two measures): Five rising notes, then a repeat of the last three notes. An exact repetition of that eight note set completes the measure, and then on to the next figure. Early on, you can think of them as arpeggios: C, Dm7, G7, etc. But by the time you get to measure 24 you've got Ab-F-B-C-D. Fm6/#11? Dm7/13? Bº7(b9)? Makes me doubt Bach had chords in mind - he was just making simple alterations to a simple melodic figure.
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