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 It is primarily of interest to outdated CAD software vendors, because that's who's driving the API specs, hence why it took so long for the fixed-function pipeline to finally be dropped.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.
The Android issue is entirely caused by shitty drivers. Pretty much all mobile hardware after the original iPhone fully supports OpenGL ES 2.0 shaders which haven't changed since inception.
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 It is primarily of interest to outdated CAD software vendors, because that's who's driving the API specs, hence why it took so long for the fixed-function pipeline to finally be dropped.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.