To be fair, the driver/library here is an RPC interface for remoting OpenGLES to their VideoCore GPU (which has its own closed firmware for handling translation between that API and the bare metal). Claiming that Broadcom is the first vendor to "open their GPU drivers up this way" feels a little misleading to me, given that what they've opened is RPC stubs and associated glue, not any actual internals.
The model here is similar to how video codec blocks and camera/isp pipelines are supported on a number of SoCs already.
However it does mean that they're able to drop full sources for the userspace and kernel bits, which is indeed really nice. Zero proprietary binaries on the Host/AP side of the world is quite impressive.