I only briefly looked at the source code, but I can confirm that the shader compiler, like almost anything else, is indeed just called via RPC into the VideoCore.
Describing this as a fully open source graphics stack is of course a gigantic marketing stunt, but I also agree that it is a major step forward. Not so much because of the technical merits of that code but because it shows how even a company that is as closed as Broadcom can move towards an open ecosystem, and because it provides a good argument to counter all the SoC and GPU companies that require proprietary blobs running in user space or even in the kernel.
It's worth mentioning that the same GPU is used in the BCM28145/55 smartphone SoCs, and I really hope that having a completely open user space implementation gives Broadcom an advantage in Android products that leads to other companies moving to even more open graphics stacks.
Since you mentioned other implementations: are you aware of any GPU besides this one that has everything including the shader compiler in the GPU itself and can do the same trick?