Where does OpenBSD's kernel fit into all of this? Isn't that, in turn, lightyears ahead of illumos?

I'm not sure in what way they would be light years ahead.

But I will say the it's probably much easier to hack on the bsd kernels than Linux.

Taking FreeBSD as an example, if for no other reason than the absurdly great documentation, tooling, and developer community.

For one, Theo De Raadt Says FreeBSD Is Just Catching Up On Security:

https://tech.slashdot.org/story/13/12/16/0121213/theo-de-raa...

OpenBSD's documentation is also known to be a tad better than FreeBSD's.

In some things, yes. OpenBSD was always, and remains the industry leader in security innovations.