He started a 51% with far less than 51% of the stake. It certainly is in his wheelhouse to pump/dump a coin, but the Steem community is blocking the hardfork he needs to lower the powerdown period. If he manages to change it we will be forced to fork away from that broken chain.
You can't dump in a situation like this. Liquidity is being drained from the exchanges. People who are buying may pump the buy price, but people who are selling are not providing liquidity either; they are selling immediately. This creates a gap in liquidity where there's a huge space between buyers and sellers. Sun will have no liquidity to dump in to.