Their website claims weathersealed. I have one coming to me on Friday.
I noticed that as well. They claim that it's made of the same material as the camera and weather sealed. I just broke down and ordered one. I doubt I'll have a hard time unloading it if I'm unhappy with it.
I highly doubt it's weathersealed since i don't see the same "sealing" foam used throughout the mk3…although maybe that's not what make it's "sealed" in the first place..i'm not sure.
As for material..it's the same material as the camera "body", not chasis..there's no magnesium in this grip.
But quality wise, you won't tell the difference between this and the original..it is by far the best 3rd party grip i've ever seen