I am also doing Santiago to Rio on the 21st September. I originally wanted my agent to book me Business class but was told that they only do Premium Econ on that route. If you look at LAN website you will find that flight LA772 deps at 1310 arr 1825 seems to be the only flight that does show Premium econ as available (especially my date).
You didn't give your travel date but as another example I looked for a flight in a couple of weeks, 15th Aug, and it shows premium economy on flight LA786 0720 - 1235 and LA772 1310 - 1825.
This is about a 4+ hour flight and it was one of my stipulations to my agent when booking that I would not fly that time and distance in Y class. and he has assured me I,m in PE.