You are correct. It's definitely not going to increase traction if you're accelerating or on icy roads.However, I do believe that adding weight in the back would reduce rear sliding.
Don't things like lift off oversteer occur because weight shifts to the front and the back gets light and slides out?
Also, as you said, FWDs have a tendency to understeer. Which means the front isn't getting enough traction. So adding weight to the back would be a big no no.
In this situation it seemed like the back had zero traction while the front had plenty. Until I was spinning that is.