He's referring to the difference between the D3D debug layer (http://blogs.msdn.com/b/chuckw/archive/2012/11/30/direct3d-s...), which can be toggled on/off for debug/release builds (or even forced on in the driver), and OpenGL's near worthless glGetError() and debug output callback. Not only does D3D come with better debugging capabilities built directly into the API/driver its tools are vastly superior to OpenGL's - though with Valve's VOGL and NVIDIA's nSight that's beginning to change.