The surprising bit is less about what they're working on and more about the collaboration itself, including the mutual and coordinated co-marketing/PR. Apple and Nvidia haven't had a business relationship in over a decade, ever since Apple stopped using Nvidia GPUs in Macs.
This sort of re-engagement and subsequent promotion isn't something that "just happens", in my experience. It's reasonably possible that this could portend additional future outcomes, such as official support for Nvidia-based eGPUs, Apple's licensing of/support for CUDA, Mac Pros with Nvidia GPUs, etc.