And of that weight, how much of it is exerted behind the seat and under the hump? I'd speculate that it's less than the weight of the tube. The rationale is that if there's no other significant structure between the shock top mounts, then the subframe, such as it is, will tend to flex. In reality, most people are cutting off the ends of two straight, or almost straight tubes.
But the point I think that people here try to make is that newbies need to understand what they are cutting off that the potential impact of the changes they make. And that is totally valid. That video was interesting tough and the fabricator clearly is thinking things through one step at a time to avoid future problems.