They used to work with Khronos but things have developed since 2008 - Apple has completely abandoned OpenCL and OpenGL, won't support Vulkan alongside Metal, and during the development of WebGPU one of their representatives let slip that there's a legal dispute blocking any further interaction between Apple and Khronos.

https://docs.google.com/document/d/1F6ns6I3zs-2JL_dT9hOkX_25...

> Apple is not comfortable working under Khronos IP framework, because of dispute between Apple Legal & Khronos which is private. Can’t talk about the substance of this dispute. Can’t make any statement for Apple to agree to Khronos IP framework.

Since the details of the dispute haven't come out it's unclear if it has been resolved yet, but regardless they don't seem to be on good terms.