That's bullshit. In practice different GPU parts support different subsets of OpenGL's shader model, leading to broken graphics depending on the GPU. This is a huge problem in, for example, Android games, where you have to test on just about every device to see what works and what doesn't.

OpenGL is cross-platform because it's legacy. It is primarily of interest to outdated CAD software vendors, because that's who's driving the API specs, hence why it took so long for the fixed-function pipeline to finally be dropped. Modern CAD vendors, such as Autodesk, render through a Direct3D pipeline.