He prefers to build in on Steem, which is only logical since he's CEO of Steemit Inc.

Apart from that, the Steem architecture has a lot of experience with content, transactions and scalability. On these aspects Steem is currently outperforming every other blockchain implementation. This is why Steem as the backbone also seems like a good idea to me.

And I don't work for Steemit Inc btw.