SmartOS is really just a distribution of illumos; so are OmniOS, OpenIndiana, etc. We are all very similar as we push and pull from our common source base.

When Oracle took the source closed again, they essentially forked their own private product. They have diverged significantly enough from everybody else that it's not really helpful to include them in the same family anymore.

tl,dr: we just call it all "illumos" now.