There doesn't seem to be anything official about how it's implemented under the hood, but the widespread assumption is that OpenGL on the PS3 is an emulation layer on top of the native graphics API, so performance tweaks are easier if you use the native API directly.
If I am not mistaken, even the engine provided by Sony in their SDK does not use it.
FireEngine if memory does not fail me.
PhyreEngine
Thanks!
I think that's because they added OpenGL ES 2.0 later on top, to make it easier for developers, but developers just preferred to use Sony's own custom API's.