I've built a few pure wing sailplanes in the past. If they were too tail heavy, they would happily turn sideways and crash exactly like the video, and this was without any edf's or tubes to blame it on. I had a 3m span one that was about 60 seconds into a flight when this happened. It seemed to be flying fine and then it just wasn't. I had trouble with spins on winch launches too. Some guys in Germany found that certain tip airfoils would keep the spins away.
Another one I made had too much twist in it and needed a lot of nose weight to get it flying without down trim. It wouldn't go very fast, but it ended up very stable this way. It was supposed to be a 1.5m slope racer, so a failure on that, but I learned something.
It's reasonable to assume that these Horten models have not all been built and set up the same, so I wouldn't expect the same behavior just because they look similar.