We know that this takes place at least after Halo 3, as there's no way as big of a battle as this could be held on the ark during Halo 3 or before as nobody could get to it before. That means that this is an entire army of Brutes, left to their own designs, meaning it could very well be a brute variant of a scarab, or an entirely new vehicle.

But wasn't the ark destroyed with the self destruct of instalation 01?

Nope. It was heavily damaged but not destroyed. One of the more recent books, I forgot which, talks about a group of scientists going back to the Ark and accidentally finding Guilty Spark, who now remembers who he was before he was a monitor. So the Ark is still very much around.

It's designed like the scarabs seen in halo 2

Its closer to that but they still look off