I think your referring to MonoGame (Open Source implementation of the Microsoft XNA 4 Framework). The Mono Project itself is basically a cross platform implementation of .Net which makes it possible to create cross plateform applications in C#/ASP.Net and many others and which Unity does use. MonoGame according to the site uses OpenTK for OpenGL rendering or SharpDX for DirectX rendering depending on which platform its running on. I've been considering using these projects myself but haven't done so yet. Anyways hope this informations helpful in clearing up any problems if nothing else OpenTK could be useful to you in the future if you decide to go with SharpDX and later want to mess with OpenGL and possible cross platform implementations.
Right now the consensus seems to be that XNA is finally dying, in its place we are left with Mono, on which Unity is built, and which is built upon SharpDX if I'm not mistaken, that would be the chain to follow depending on the level of abstraction you want to work with.
If you like low level take SharpDX, if you want some wrapping and multi platform support but don't want a high level abstraction use mono, and if you are fine with taking only gameplay logic within a highly abstracted framework, Unity is a very comfortable engine to use.
PS: Posted mostly to try and clear up any miss misunderstandings.