The easy part is that he mounted a Rolleiflex/Rolleicord Plate Adapter, which was originally meant for wet plate photography. But getting the Instax film into the plate adapter and exposed is where things get tricky. The original Google Translation of the page is a bit hard to parse, but according to PetaPixel, the Instax has to be loaded into the adapter in a darkroom or film bag. The black card protecting the film has to be manually removed before exposure, and then replaced. And to actually develop the film, you either need to put it back in the Instax and take a totally black shot to push it through the rollers and activate the chemicals, or else crush it manually but risk an uneven developing process.