I have no idea as I always thought it was just a large sized integral that had a dark sleeve so that you can take it out of the processor...
I could be wrong, but I think the Instax is also exposed through the rear of the film, not the front like Impossible, or regular integral film? This, I was led to believe was done for usual Japanese efficiency reasons - it simplifies the manufacturing process, uses less layers than Polaroid integral style (including Impossible of course, and making both sheet & pack thinner), and was supposedly easier technically. The chemical 'dark slide' in Instax going opaque without the need for timing, and merely when ejected, vs. the Polaroid chemical 'dark slide' going transparent as the print develops. Knowing the problems Impossible had with getting this part of the process right, it sounds reasonable. Skj.