Looks like it's a fork of OpenSolaris[1], which is pretty cool - I didn't realize there was a Solaris variant still under active development.
There are other variants under development. Illumos probably is the largest one, SmartOS another.
The OS is packaged the same way Linux is, with different distributions maintained by people outside the kernel devs.
I think it's not ideal to use the term "distribution" for their downstreams like SmartOS etc. because it reminds people of all these GNU/Linux distributions, but a better term doesn't immediately come to mind...
And have you tested recent Linux w/zfs?