STOL a military animal....que..?
That's the general story.
Robertson STOL conversion
I have no specific familiarity with the Robertson mods. However, I suggest that what they do is reduce the stall speeds which, in turn, reduce takeoff and approach speeds while maintaining the usual certification margins above stall. This then reduces runway requirements. Fine to market that as STOL but it is not what STOL design is, per se.
I am happy to be proved wrong in respect of the certification .. but that will require a copy of the relevant approved POH which will tell the story in some detail.
Real STOL, as the military do things, is a different animal which operates much closer to stall and accepts a considerably higher probability of mishap in the event of something going awry. The benefit is a shorter distance requirement. Acceptable for some operations in the military but not for routine civil operations.
in the highlands of PNG
Now, I don't have any information regarding the specifics for the aircraft Type in PNG ... but do keep in mind that the old DCA rules were a tad different for developmental operations in PNG ... so it may be a case of comparing apples and oranges. It's so long since I had a look at the old rules .. the poor old memory is not up to recalling the details now.