I wouldn't want to be standing on one. You would be putting a hell of a lot of faith in the stength of a rung. Two guys and the weight of the platform.
Rungs are designed to take downward force, the weight from the outside leg of the triangle is pushing sideways. I think it would be a safer platform if they were not using the actual ladder to form a part of the triangle. If they had one more leg, so that the platform itself had a triangular base it would be safer. This would tansfer the sideways force to all the rungs in contact with it.
I'm not a structual engineer, but I think that most of us that have spent years working off ladders, become experts in understanding how forces work. We instintively apply the laws of physics every day. forces and friction being the most important. They keep us alive. Dai