Any open source replacement for the Raspberry Pi GPU driver is going to rely pretty heavily on reverse engineering too. The code they've released runs on the VPU, a custom-designed scalar processor within the VideoCore IV that Broadcom still haven't released architecture documentation or a toolchain for. Without a whole bunch of low-level reverse engineering (much of which was already in progress) it won't even be possible to compile the stuff Broadcom have released. Without working VPU code you can't even boot the Pi, let alone make use of graphics.