It isn't, mainly because it solves classical physics, i.e. Newtons equations. In contrast, your linked paper discusses numerical relativity, that's an approach

It isn't, mainly because it solves classical physics, i.e. Newtons equations. In contrast, your linked paper discusses numerical relativity, that's an approach to solve Einstein's general relativistic theory of gravity. While nowadays we do have a mathematically kind-of-similar formulation (an initial value formulation of a partial differential equation system which can be solved with similar techniques as classical mechanics -- so indeed with similar techniques as the "physics engines" refered here to), the bleeding edge research is at another scale. We use high performance computers to solve a large and complex system of equations (typically several MB of source code defining the system) in very large computational domains which take GB-PB of main memory on thousands of computers. It may be worth saying that numerical relativity played a key part in the last Nobel prize for the detection of gravitational waves. Nevertheless it's nice to refer to this analogy.