The service doesn't seem to give proof of ownership, just proof of existence at a particular point in time (ie. This document was submitted and verified at 12:13PM PST on 2/18/2015, so it must have existed at that point in time).
Ownership of content could be supported in the sense that you publicly annouce its creation first (blockchain is public) with no other previous record of it anywhere. Of course if the content is not original work then it should be traceable in the past thus it would be completely pointless to stamp it on the blockchain.
actually you can't. But there is nothing to gain in doing so. This seem to work only if the content is original work