To respond to the title: yes, yes they are, if you consider how many live in infura or an AWS geth instance and/or quorum by JPM. Doesn't mean its not a better means for data security...just that yes, the publisher of said smart contract can just replace or change to contract if they really wanted to if it were not part of the mainnet. There are many infura nodes though that are part of the mainnet, so that's unlikely unless some company just forced all users to their own ETH network.