They will use Illumos with Bhyve, @bcantrill said it in a podcast just a few months ago. I have linked it somewhere in my comments (look at my profile).
Is that Illumos or SmartOS or are those considered interchangeable these days?
Illumos is the project that multiple distributions build on from what I understand. In a way, it could be likened to GNU/ Linux as Illumos probably contains not only the kernel but other tools and libraries as well. There is e.g. omniosce.org, perhaps Nexenta and Joyent/ Samsung SmartOS.