No (unfortunately) it's NIH and nothing more.
That's a huge difference to what NIH means, which is that just because I didn't make it, I'm not going to use it, I'll make my own version instead.
They couldn't ever use the original version of things because they're legally bound to not do so. The kernel has had a long standing policy of not keeping APIs that either they or userspace aren't using to reduce their burden and the case of keeping that API just for ZFS since literally nothing else in the kernel was using it wasn't a good enough argument to keep it. ZFS on Linux has now worked around the removal of the API.
I'm just curious: how did they do that?
Isn't that going to kill performance?
That also assumes though that the cost for handling the vector instructions in-kernel isn't higher than doing it without them. I believe that was part of the reason that the kernel started removing this from within drivers and other parts to begin with, the cost of saving all the floating point registers and other state is actually surprisingly expensive. So it may be a complete wash in the end anyway. If the kernel is no longer having to worry about saving those registers it can mean that the userspace doesn't pay that extra cost and now your compute ability is better for it without seriously affecting any of the rest of the system.