They used a vinyl plotter to mirror cut the vinyl then applied to the backside of the top sheet. So when they top sheet is flipped over the image is forward facing. It's a good idea to be honest.
I'd be worried about delam if it's really just vinyl. That adhesive isn't permanent, and epoxy doesn't adhere well to vinyl. I could see doing a mask and then painting?
Could it be some perforated vinyl film, so that the epoxy can bond to the topsheet via some small holes? Just like there is perforated release film in vacuum bagging...
Print shops have screen inks that are on films that can be die cut like this, they make teeshirts this way too. My teeshirt print shop has this stuff
It's basically just like glorified iron on graphics, except the heat presses the screen ink onto the piece, it's apparently more durable than screening.