Think that was in part a reference to the origins of the Sidhe themselves. From what I reckon they started out as beings wizards used to help perform magic (like the deal Harry does with toot, except they ask for them to use some of their innate magic) gaining power and stature from the deals and the perception of them as wielders of fantastic power, ergo gaining it by proxy. Eventually the original beings becoming mantle's of power of course. Or conversely, the ones who repeatedly summoned them gaining such power from it, banner like into an actual power base. All conjecture of course, but it seems accurate in the druid into wizard transition.