His track uses the <Point> KML command, which has an associated Altitude co-ordinate.

Alas, there's no such co-ordinate for the <GroundOverlay> command, nor its internal <LatLonBox> command either.

And I suppose it's fair play, really... the computation involved in 'stretching' an overlay in realtime, whilst continuing to project it properly over the globe's surface, is asking a lot. And stretching it is what would be needed, if it could 'float' above the earth's surface, and then be rotated in realtime as you zoomed around the globe.

With points, it's a lot easier to do... even with lines (which are really just 'point-to-point' computations), it's manageable. But not full blown bitmap overlays, where the points can run into the hundreds of thousand!