I couldn't find any details about the migration on either links but it looks like they make massive use of Nvidia-specific features, so even with exactly the same performances it would make total sense to use Cuda just because the tooling is more mature.
Thanks. But I don't see how this fits with your previous statement:
> Badly, OctaneRender had moved away from Vulkan into CUDA, because they found out that Vulkan compute wasn't at the level that they wanted.
They mostly talk about Vulkan+Cuda interop which isn't really supported, and they explicitly said they consider rewriting everything using Vulkan to get rid of this issue. So from what I understand, they are still pretty bullish on Vulkan, but it will require a lot of work and it will take some time (“but probably won't be this year”).
Optix doesn't do Vulkan and I doubt that NVidia will ever bother, and OTOY most likely will rather use they resources elsewhere like the new Metal render.