He takes a photo of the landscape, then goes home (or wherever), and takes a photo of the jar (I'm guessing painted black) on a white background. The white background over-rides the landscape on that section of the image (the white 'blanks' everything else out), but if the section of the first exposure that falls where the lid is isn't sky, it'll show on the lid. I think....
He does a double exposure, because the only other way to do it is with digital editing or double exposing the print with two different frames. I don't optically print, but I would think it's a lot easier to do it in-camera then at the enlarging stage, and he has the ability to print it on a 'digital' printer.