I would say its debatable, if its a myth or not, but it needs to be in the told/read in the right context.
When removing a large part of the feeder roots, in warmer conditions, those roots cannot provide enough water, to keep them stiff and healthy. (transpires more then roots water intake)
When removing a part of the foilage, it transprires less, and can use those healthy leafs to produce new sugers to create new roots.
If it needs to be the same % i would say that is wrong to assume. It should be determined by the health of the tree, and the conditions / locations of the whereabouts.