Safe? In my opinion, yes.
I think it's foolish to think that even when you cover it with silicone and peat that you are completely sealing the foam from the water column of the tank, if that's what you're thinking. Polyurethane based products (greatstuff, types of gorilla glue) actually catalyze more quickly in the presence of water. As with anything, it can breakdown over time. A long time, likely longer than the length of time the viv will be set up for.
The one thing I'm not 100% sure about is the coloring agent they used to make it black. I assume it's non-toxic because it's safely used in ponds that have fish, amphibians and plants.
I would feel comfortable using it. You might not get very good plant growth because of how GS tends to form that glossy outer shell that most people carve away, but I have no experience with that.
PS make sure it's hella cured.