Individual smart contracts cannot be altered, but they can be replaced by newer versions (append only). The old versions remain in the blockchain. So the immutability aspect is that if a smart contract is changed, you can always look at the old version to see the differences, as you mentioned.
All blockchains are databases, but not all databases are blockchains. The most important requirement for a blockchain is that a) each block has a hash b) each hash is a component used to determine the next hash, and c) the previous hash is referenced by the current block.
That makes it a blockchain. Does this alone make it decentralized? No. Is there real consensus? Not yet. But the Steem Engine sidechain qualifies as a blockchain, by this narrow definition.
so i can call steem engine a dex? Ive been calling it a semi dex or quasi dex
I always referred to it as an aspiring dex, you know, chugging away at the goal but still waiting on tables on the side...