If you're wondering what plane, it would appear to regularly be the dreamliner (http://www.traveller.com.au/airline-review-latam-economy-san…) and they're also talking about upgrading business seats (https://www.ausbt.com.au/latam-reveals-new-business-class-se…).
However, on to the deal. This can often be 4 figures, but for certain dates, as in the example link, you can get it from $735. Return. In business. You pay that much in economy at Christmas!
For more deals like this, visit https://www.beatthatflight.com.au or https://www.facebook.com/beatthatflight
LATAM J is usually around 700-800 return. so while a decent price, it's not the amazing bargain the OP makes it out to be…as for the potential upgraded J seats, that's not happening in the near future. the new seats are for the newly ordered A350 and 787-900, not retrofitted to the existing 787-800 which are relatively new.
I've flown this a few times, and the cabin isn't the flashiest. the best thing about J on LATAM is actually the Qantas lounge access… :P