Suggesting that Apple silently correlates and feeds back iPhone GPS data to Skyhook's access point database is speculative, at best. Occam's razor says Skyhook is using off-the-shelf hardware to identify access points at a distance, especially if your island in the middle of nowhere affords clear line of sight.
Your wifi only being on sporadically isn't dispositive either. I took an Airport Express to a hotel once for a two-day conference, and for months afterward my iPhone thought my apartment in Seattle was in downtown San Diego.
Your Occam's razor has access to a pretty nifty shelf.
I don't think you are going to find off the shelf gear to receive an Airport express through 12 inches of log, then across 1 mile of water with the surface of the water in the fresnel zone, I'm only about 10 feet above water level and the nearest land road is about 20 feet above water level. (The multiple paths from reflections off the lake create all sorts of cancellation problems.)
Even more unlikely, if they could sense my access point, they would still have to work out a vector and range to me since the nearest roads the other way are over the horizon. The road does not afford a sweep for meaningful triangulation. We are firmly in "zoom and enhance" territory now.
It doesn't have to be Apple sending data to Skyhook. I suppose any app with access to the location data and MAC address could do it. Or maybe Skyhook sent out a boat. Or perhaps someone manually sent the AP coordinates to skyhook.
Your wifi only being on sporadically isn't dispositive either. I took an Airport Express to a hotel once for a two-day conference, and for months afterward my iPhone thought my apartment in Seattle was in downtown San Diego.