I have no doubt that the rearmost part of that engine cover is semitransparent.
Look at the picture posted before. All those so-called reflections are perfectly aligned with the other side (Check the blue lines).
In the pic below, all those parts visible on (or through) the rear potion of the engine cover is also in perfectly continuous with the background objects, not even a slightest distortion.
Same situation below...
In the pic below, right vertical part of rear wing is also visible through the engine cover. Also note that the reflection of rear wing on the engine cover doesn't continue through the rearmost part.
Same situation. Check the rear wing reflection pattern on the engine cover and check the right vertical fin of the rearwing.
Again, even the sky can be seen through that part. If it's reflection we should see it on the front parts of the engine cover too.
So, as you can see, not only at one angle, but at different angles the situation is same too. It's only at the rearmost part, not at the whole engine cover. I don't know why we can't see the "1" of Mobil1 and Mercedes star at the other side, but we can see all the other things that are behind that part. And while we can see reflection on anterior parts, we don't see any reflection at that part. Under these circumstances, I think it's semitransparent. And that bears a second question. If it's semitransparent, then it's not made of carbonfiber. Then what is it?