Indeed, that's the key difference. Steemit coins inherently require trust in steemit, since without the website's existence, the coins have no use at all. As everyone is implicitly having to trust steemit, there's no benefit in running a blockchain.

As you say, the key point of blockchains, like bitcoin, is that there is no central trusted entity. So, like steemit has done, if you add a trusted person or company, the main feature of the currency is wiped out and there's no point in having a blockchain in the system.