While he's right that it's not as big an issue as ZFS fanatics make it out to be, it _is_ a real issue and they're not just pulling it out their asses. There are a number of studies that actually measured the error rate, some of the bigger ones being done by CERN [0], NetApp [1] and IA (I think there's meant to be a talk or something to go with this one) [2].
ZFS certainly isn't a magic wand you should wave at anything and everything and it doesn't replace backups but it does make the chances of something going wrong undetected much smaller and even though the chances are small to begin with, there are times when you just can't accept it at all.
Its not backed up until its at least on an external system, ideally in triplicate off-box, off-site, and cold storage. Cloning and replication makes it easier to backup but is no substitute.