Steemit would have to open-source their code, making it available for any programmer to inspect the code, but also copy it and redistribute it. The whole idea of bitcoin/blockchain is to provide transparency and to provide it freely and openly for anyone to inspect, use, copy, modify, improve, etc. So because Steemit is not open-source, we have no idea if they really are using the blockchain, they're responsible for proving it but in general it's almost impossible to prove unless they release the code, which means they don't get to privatize Steemit and make profits from it. Which is fine, but it's just not as transcendental as Bitcoin or any open-source software