It may be using a pair of "E" or "U" cores or pot core style ferrite / powdered iron cores with a coil in each, so in effect there are two large electromagnet poles against the metal at each side, with the windings on those spaced away from the metal to minimise energy loss.
In other words, energy would be passed via a magnetic field directly through the metal and with minimal electromagnetic induction effects in the plane of the metal.
As long as the metal is quite thin in relation to the size of the core polepieces, fairly good magnetic coupling could probably be achieved.
I can't think of any way of making it work through a metal panel other than lots of ferrous material to guide the magnetic field.
[Other than cheat? Use a panel of plastic that has a metallic looking finish....]
These are something like the style of core I was thinking of, one at each side with the coil in the back half of each; I can't say as a fact they would work, but I believe a setup something like that would give the least energy loss and heating of the metal panel.