Illumos is a fork of OpenSolaris: https://wiki.illumos.org/display/illumos/illumos+Home
Basically ZFS is Solaris (Oracle), everything else is OpenZFS. And every Solaris-like OS is at best just going to be an OpenSolaris (or derivative) fork.
Yes, one devision of Oracle owns, maintains and develops ZFS. But one of the many other divisions (maybe for historical reasons, maybe because it was acquired and never migrated, ...) might use btrfs, and it makes sense for them to pour manpower into it, even if that could be perceived as somewhat of a competition to ZFS.
My main point is that big organizations naturally tend to do things that look conflicting from the outside, just because they are too large to be efficiently standardized.
I'll just leave this here for people to watch: https://www.youtube.com/watch?v=-zRN7XLCRhc