It's the successor of OmniOS by OmniTI which is based on OpenSolaris [1]. Tobi Oetiker (of RRDTool) and other have adopted the project. See the "About" page which is unfortunately hidden in the Site menu: https://omniosce.org/about/about.html
Probably because they're both based on OpenSolaris
The SmartOS and OmniOS folks work together from time to time. For example, I believe Bhyve was first ported from FreeBSD to OmniOS before it eventually came to SmartOS.
Other way round. Like LX, into SmartOS first, then brought across into OmniOSce. But yes, OmniOSce picks up a lot of SmartOS features, whereas some of the other distros are vanilla illumos.