Shrug; you're talking about reverse engineering hardware implementation from the details of the transferred data. Source access is hardly required for that, and the Nouveau driver we're talking about is an existence proof. And in any case all modern GPU manufacturers except NVIDIA ship GPL kernel modules and aren't leaking "proprietary IP" to each other. This isn't an issue of secret sauce in the kernel, sorry. NVIDIA just don't want to spend the effort to conform to someone else's architecture.

Even NVIDIA takes this approach on their mobile integrated GPUs on Tegra (GPL'd kernel driver).

I wish they'd do so on the desktop too. Having the kernel driver (which really does not need to be all that much code) be open source makes it massively easier to upgrade the kernel around the GPU driver.