What good is a fault tolerant file system if it isn't tolerant of faults?
Any time you read about a product that guarantees perfect fault tolerance, there is always a list of constraints that must be met for that claim to hold. You probably won't ever see this list marketed, but it's there somewhere.
I haven't looked into this, but it sounds like ZFS is fault-tolerant given a system model where data can change once it's on-disk, but otherwise system components are fail-stop. So if you ask a hard disk to perform a write barrier and flush its data to disk, the disk will either do so and report success, it will issue an I/O error, or it will catch on fire. Any way, ZFS will handle the situation correctly.
Of course, the immediate next question is whether that failure model is realistic. Turns out it isn't. Some hardware will report 'write barrier complete' when it still has unwritten data in its buffer.
If you can't count on your hard disk to flush its caches or even order writes correctly, I don't think it's possible to build any fault-free file system on top of the drive.