Why is the 2D X11/DDX driver and kernel module closed source?

Surely you could at the very least provide source for the DDX.

How do you figure? 2D accel requires use of the 3D engine (there’s no dedicated 2d hardware on modern chips), so even the DDX highly likely contains stuff that Nvidia doesn’t want to/can’t release.

As for the kernel module, if the driver is structured in any way similar to the open drivers, then the module contains their memory manager, which also falls under “secret sauce”.