I don't think this is true (at least it wasn't when I flew IPC-SCL). IPC is a domestic flight into SCL, but from what I was told, it goes through a different arrival check as other domestic flights. Our 767 was parked in the international area of the airport (even though this wasn't one of the PPT-IPC-SCL flights), we skipped immigration checks, but we were taken to the customs area where all our bags (including carry-ons, we had no checked bags) were passed through x-ray machines. I was told this was to prevent certain pests from entering the mainland, but I don't know if this is true. Maybe there was a way to go to the transit lounge directly, but if wasn't obvious, and we were terminating at SCL that day anyway.

Going the other way (EZE-SCL-IPC), I asked everywhere if it was possible to just transit, and was told no every time, so after immigration, we had to go through customs, then out and back up to the international departure area, after having our boarding passes stamped "Isla de Pascua" so we could access the lounge. IIRC, because this flight went SCL-IPC-PPT it was sort of boarded from both sides, with those going to PPT or with lounge access on the Int'l side, and those going to IPC without lounge access, on the domestic side.