The PS3 is OpenGL based. Specifically it's OpenGL ES with some extra proprietary Sony stuff. The important point is that the engine is not directly tied to any low-level rendering API and clearly has enough abstraction to be ported to multiple platforms with a reasonably low cost of development.
Your first comment isn't entirely accurate. While the PS3 does support PSGS, which is the library you are referring to, engines like FrostBite use the LibGCM interface, which has very little similarity to OpenGL.
The PS3 is not OpenGL based, it provides an optional OpenGL-like layer should titles want to use it but rarely do any these days use it. Frostbite pokes the GPU FIFO directly using the libgcm.
