Metal is racing against Vulkin as well, because Apple needed to have their own standard they owned. Except Metal has zero support outside of Apple products.
Besides, what Metal represents is Apple's attempt to have yet another vector for their infamous walled garden and vendor lock-in strategies.
Further, the article and the website are intentionally and blatently biased. Quotes like this showcase their extreme bias:
> One can be righteously indignant that Apple isn't subsidizing everyone else with support for their platforms, whether CUDA, Vulkan, or even Android. But such emotions won't have any bearing on the final outcome of who wins and who loses in the market for developing and commercializing the graphics technology of the future.
The absolute derisiveness of open standards coupled with their insults against those that would showcases quite well the position of the article, its authors, and the website itself.
Also, Vulkan is quickly becoming the lingua franca of graphics APIs. Vulkan can be implemented on top of Metal, which means that developers have next to no reason to implement Metal directly.