This is incorrect. The effort it took to enable Vulkan was relatively minor. The PR is short and to be honest it doesn't do much, because it doesn't need to.
(I mean, the missing work should not be much, but it still has to be done)
Maybe I should clarify that I'm not saying that the effort to enable a new backend is substantial, I'm saying that my understanding of that comment (the one you acknowledged made a good argument) is that the maintenance burden of having a new backend is substantial.
But now suddenly what I said is not just an argument you disagree with but is also incorrect. I've been genuinely asking for several turns of conversation at this point why what I said is incorrect.
Why is it incorrect that the maintenance burden of maintaining a Vulkan backend would be a sufficient explanation for why they don't want to merge it without having to appeal to some sort of conspiracy with Nvidia?