I don't have any experience, but looking at pictures, I'm not sure why it is being called a bivy. Looks like a double tent. Perhaps they call it a bivy to manage expectations. It doesn't look very tall compared to most tents and it is not freestanding.

4.2 lbs seems a bit heavy for what it is. I guess they don't use high end fabric.... so you could say 4.2 lbs is pretty good for a tent that cheap. I would guess a similar tarptent would weigh a little over 2 lbs... of course it would cost 5 to 10 times as much.