Any chance you can provide another photo of that piece?
That certainly isnt plywood. It is some sort of OSB panel, but hard to tell for sure from the one photo.
Here are two other photos. My guess is that it is some kind of OSB, but I'm sure you could email fiberine and they would tell you. Whatever it is, the roof rock solid, and I would have no concerns about putting a cargo rack and a few hundred pounds of stuff up on top of my van.