I'm guessing he means that RAID is designed as a technology to increase the availability of your data and the person in this story is using it as a backup solut

I'm guessing he means that RAID is designed as a technology to increase the availability of your data and the person in this story is using it as a backup solution. Google 'RAID backup' to see why it's such a bad idea to use it for that purpose. He specifically used cp because he was worried about data loss from using something like dd. But if he'd been doing regular backups to tape, he could have just restored the backup from tape with no fuss or problems. It's only because he was using tools that aren't suited to what he was trying to do that he ran into problems.