The flight is domestic. You will go through security into the domestic airside and will not have access to the latam lounge.

The part abount departing from the international airside is only true in part. You will most likely board on gate 20, which is sometimes used for domestic and sometimes used for international as it is the "moving thereshold" between both airsides. For all pratical purposes, you'll always be on the domestic side (no lounges, no duty free shops, etc)