It's not invisible metal..... you can see it.
Look onto windscreen from outside; there is a thin conductive metal layer in the windscreen that gives it a noticeable tint. How do you think electric current could otherwise flow through it (which heats it)?
This does absorb some signal - not only GPS but also other electromagnetic spectra. Hence VW have bundled it with the advanced telephone system.
VW are the first to use this wirefree technology to heat screens iirc, but it has been around for a while. My brother's mid-2000 C4 had a 'climate' windscreen (not heated) that took out IR to stop interior from heating up (or at least less) and that was pretty good at lowering the signal so it would take his handheld sat nav ages to get a lock and frequently drop out.
Ford's system has some more blank space in between the heating wires, so potentially this might allow more signal in. I never tried a handheld sat nav in a Ford though.
As I said, usually GPS (or GLONASS, BeiDou, Galileo) reception should still be OK unless the weather is poor (very overcast, thunderstorms, etc) or you drive in a location with tall structures (skycrapers, industrial plants, mountains, etc).
