I'm talking about filesystem level support, not block level support. FDE is block level and of course will work on any supported filesystem including btrfs.
What btrfs doesn't do is support encryption inside the filesystem a la fscrypt - where we can encrypt specific directories - or ZFS encryption, where we can encrypt specific filesystems inside a pool.
If I understand btrfs right, supporting encryption/verification on subvolume or directory levels would allow splitting the system so only part of it is encrypted/verified (so we get Poettering's performance gains), while avoiding space issues between the subvolumes.
Also, using only one password is easy on fscrypt style systems, though this may not suit everyone's threat model.
