As a reminder, the same (closed-source) user-space components for OpenGL / OpenCL / Vulkan / CUDA are used regardless of the NVIDIA kernel driver option with their official driver stack.
Usually this is done for licensing reasons. They probably don’t want the old code caught up in the open license they’re shipping the new driver under.
My understanding is that the new open driver separates proprietary code into a black box binary blob that isn’t distributed under an open source license. I’m guessing that they’ve been very careful not to include anything they want to keep closed into the new open driver, whereas the old driver wasn’t written with this separation in mind.