Sir, this is complete, utter bullshit.
DirectX gets almost nothing “much earlier”, because it has no extension mechanism. With DirectX you are stuck with the latest version. It has obvious advantages, but early features are certainly not amongst them. Think what you want about the ARB, it does release and releases often.
As for the documentation being terrible and vague, that's pretty uninformed, too. Every extension is fully documented and the vendors know precisely what needs to be implemented. There is no Direct3D equivalent of the 600-page OpenGL specification. The DirectX documentation is a programmer’s guide, not a specification. Every single version of the GLSL standard comes with a full grammar of the language which lets you reimplement a parser or compiler. There is no such thing as a grammar for HLSL (the D3D equivalent). What Microsoft calls a “grammar” for HLSL can be found here and anyone not even in the field of graphics programming will immediately understand how much of a joke it is compared to this (pages 166 to 174).
(Source: I work on Windows, Linux, PS3, Xbox and mobile game engines)