The whole point of D3D12 and Mantle is to decrease driver overhead, improve multi-threading support, and increase the number of per-frame draw calls that can be made. Those are all CPU performance improvements so I think it's safe to say that CPU speed is quite important or else AMD and Microsoft wouldn't be going through the trouble.
The blog states that if the Mantle specification is adopted by other manufacturers, particularly on mobile, then it will become the new cross platform standard and OpenGL will cease to be relevant. 67% of native speed is astronomically slow in the world of real-time games where improving per-frame performance by 3ms is considered a huge win. Getting all of your nice special effects and post-processing implemented while still running at a smooth 60fps is very difficult - especially on mobile.
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.
He's referring to mindshare among professional (game) engine developers - those people who try to get maximum rendering performance out of multiple platforms. His statement is totally on target for reasons I enumerated with a blog article on this subject back in December: http://inovaekeith.blogspot.com/2013/12/why-opengl-probably-...
As for WebGL Apple intentionally disables WebGL support on iOS - except for in iAds - so game devs can't circumvent the app store. Since WebGL is needlessly restricted to the feature set of the OpenGL ES specification its usefulness is severely limited. WebGL also has many other problems such as the fact that JavaScript is slow as hell.