Sure, that's true. steemit.com is a centralized website run by a company called Steemit, Inc. The Steem blockchain actually doesn't belong to anybody which makes it 'decentralized'. Still the whole vision of decentralization doesn't work if I can't use the coin as I want.
Exchanges are real gatekeepers.
If the website gets shut down since it is centralized the steemit coin will be worthless.