The cited article has a. nothing to do with CoreOS, and b. contains very incorrect information about btrfs, not just a matter of also being outdated. e.g. "btrfs does not allow you to organize subvolumes" has never been true. The article must be old because send/receive, deduplication, and preliminary raid56 support have been there for a while.
Damn ironic, given that Poettering wants to build a whole OS using cloud containerization and BTRFS. And CoreOS depends heavily on Poettering's systemd (to the point that they have built their own containerization system on top of it).