I checked it.
When I click on the "Deposit Steem" link, I get a "회원님의 자산보호를 위해 일부 암호화폐의 입출금 재개를 보류하는 점 깊은 양해 부탁드립니다." message, which approximately means that "we approciate your patience or ask your deeper understanding of reserving the resumption of depositing/withdrawing some cryptocurrency for your asset protection."
So depositing Steem on Bithumb is currently not available. You have to have it on the exchange to sell it.