> BTRFS was developed largely concurrent with, and in direct competition to Sun's ZFS

ZFS started development in 2001, and released on Solaris 10 in 2006. Btrfs started development in 2007, and the disk format was stabilized in 2013. ZFS has continued to gain features over the years, but I would say btrfs was largely developed after ZFS and still hasn't reached feature parity with the original release of ZFS due to the RAID5 issue.