Out of curiosity, what are your qualifications to be making these statements? They're rather bold.
You have flat out tried to tell an aeronautical engineer that something is behaving unrealistic (and used a video as proof), but he tried to explain to you what was actually happening in the video and why that is NOT realistic to normal ops of the aircraft sans people moving around in it with a high AOA and ever-changing CG.
Furthermore, numerous REAL WORLD Islander pilots were on the test team providing feedback and signing off on this before release.
To me, this seems more of you not really knowing this aircrafts characteristics, but instead just guessing/assuming.
I'm all for you having an opinion on aspects you personally don't enjoy (like sounds or textures), but if you're going to trash talk flight models at least back it up with something more valid than a parachuting video.