I've inspected the CR2 file and can make the following observations.

The hierarchy is indeed  hip > buttock > thigh but there is also a bone named internally as "rGhost", and externally as "Right Thigh Operator" which is a child of the hip. I believe that you had not made any mention of this previously.

The x,y and z rotation channels in the ghost are slaved to the x,y, and z rotation channels of the thigh.

The chain of command therefore goes as follows:-

Posing the V3 thigh will cause the dress thigh to move, (which does nothing to the dress). However it will cause the ghost thigh to move via the ERC code and the ghost thigh being a child of the dress hip causes that to move.

V3 thigh > dress thigh > ghost thigh > skirt

So it works using ERC code and additional bones, (body handles), much as I surmised in my earlier post.

It is an elegant solution but it is not the only solution. For example I've seen these set up using the thigh to control a morph.

I suggest that your next step is to try and understand the principles of what is going on rather than restricting yourself to this one solution which while it works well for this mini dress may not be the optimum for something full length.

Hope that helps.