I had always understood hat the gunwale was really strong. Using a gunwale hung trolley for storage was good because it has nothing holding water against the hull. and I understood that for trailing it was good because the gunwale was really strong.
I think they are to overcome the problem of trailers usually not really having very good suspension. Large jolts transferred directly in to the gunwales on the road are more likely to damage the hull than a short period resting on the hull.
The video is all about decreasing stress on the gunwales while the hull is on a dolly by transferring the weight to a strap/sling. This concept is counter to the Seitech solution which aims to get the weight off the sling and onto the gunwales.
Puzzled as to what really IS the best solution
Steve also doesn't say much about trailering the boat at highway speeds. I n this context, I don't think this sling is compatible with a Trailex or Kitty Hawk trailer. Perhaps Steve is aiming solely for the UK market?