If they overran the runway by five meters (and landed on 15), they should have still been on the short gravel overrun/area that never got paved. Or did they overrun the overrun?
Rumor (when the 737s first started going in) was that the 737s were operating underweight to make it work, not sure if that is still the case or not. While they strongly prefer landing on 15, they do occasionally land on 33. They regularly depart from both runways. It most definitely is not a one way runway.
Last I heard, there were two -200s, Trigana had up to three 737s (I assume all -300s) and Cardig Air had one 737 of some variety (-300 or later) operating to Wamena.