DirectX is a collection of interfaces to various bits of hardware. Graphics, sound, and input are all part of DirectX. Mantle is just the graphics side. Mantle is competing for the graphics side.
It is a shame it was not done in conjunction with NVidia. Both companies have complained about the overhead Microsoft keeps adding to the graphics side of DirectX and, at one time, were looking to work together to establish another API. Obviously, that fell apart.
I am not a fan of the direction Microsoft has taken with DirectX, but without NVidia support, Mantle is going to be relegated to the "neat tech" corner.