Actually, when I wrote that etude I thought about that and hoped somebody would someday ask that question. So being the first one, you are the proud winner of an all paid vacation to Wala Wala (wherever that may be).
The 1-9-1 over a triplet is technically a 1-2-1 but since it takes place immediately after an official “9”, in this case the ear has already identified it as an upper extension. In addition for reading the intervallic script it would add confusion to immediately call it a “2” after just playing it as a “9”. So, it’s more of a convenience decision or call it exception.